To be fair Beechcraft are far better built and rather more “heavy duty” than the Cessnas and Pipers one sees in the light aircraft scene, so that gives you a chance. The speed thing is a bit of an obsession with pilots, but in real life usage it’s better to get there in comfort 5 or 10 minutes later. On short sectors within the UK or Northern France we found a 230kt C90 was only a few minutes behind a 405kt Citation. Part of this is because the jet never gets to fly at a proper cruise altitude. Remember below FL100 it’s 250kts indicated maximum.
the PC12 has only seen one minor evolution
There have been several milestones. It has probably more to do with the fact, that a PC-12 has a life beyond carrying around “rich” people. Be that air ambulance, or cargo hauling. Also the inflow of new airframes is AFAIK somewhat restricted. Pilatus has committed resources to PC-24 program and production of military trainers.
Speaking of twin-turboprops, has that requirement to calculate with accelerate-stop distances under EASA gone away? If not, it would prevent you from going into smaller airports where PC-12 or TBM could go. Possibly a significant restriction. I’m curious how the PC-24 will perform in this regard.
Only thing missing is a potty one can use
You can have a chemical toilet instead of the middle left seat. But I don’t know if it’s useable enough for you. 
My experience is that the newer and better equipped the plane, the more the maintenance and depreciation costs
My experience is precisely the opposite. Our business has operated turbine types for years and the change from a 2005 aeroplane to a 2014 aeroplane of similar type has been very successful and not just because of the warranty, but because things haven’t gone wrong. The whole concept of these old turbine twins is fine so long as you are either lucky, or happy to run around carrying some defects. For example old weather radars are notoriously unreliable, and $30k goes nowhere on a repair. We spent more than that on persistent TCAS faults, it goes on and on. We work on the basis that every system should be working 100%.
My comments were on a rather different level, for instance (in the extreme case) comparing a 1940s aircraft with no electical system whatsover with a newer retractable single… The former is a lot less hassle and cost to own and operate, particularly if you like everything on board to operate correctly (I’m that way too)
I’ve also observed that new factory fresh singles take a lot of debugging, often with as much expense and hassle as buying a used plane and bringing it up to speed.
I could almost write that my experience is that a new SEP will cost a lot less in unscheduled maintenance than say a 20 year old version of the same.
The reason I can’t actually say that is because I had a mass of early avionics failures – worth ~ €50k-100k (!!) in list price value, replaced under warranty, and believed to be due to the aircraft having been built with a lot of stuff returned by customers with intermittent issues, and released to service by an avionics shop which didn’t check it properly.
The main reason why the 20 year old one will cost more is that lack of maintenance is unfortunately the norm rather than the exception. Especially in correct lubrication, and that translates to having to replace airframe parts which are silly-expensive no matter what aircraft type it is. If you maintained it properly, hangared it, and ACF50-proofed it, it should be fine at the 20 year point. It will still be shagged in “detail” e.g. lots of chewed-up screw heads and even some stripped threads, but that is just how maintenance goes… too much use of power screwdrivers.
I think there is a right time and place for buying new: an owner who is new to aviation ownership and is getting something non-trivial. You may well get the €50k or so (the extra cost of buying new, over 2 years old) back, on say an SR22.
Or, obviously, somebody who just wants a new plane, for the same reasons people buy new cars.
But there is a huge gotcha with buying new: you are going to be totally on the case, making sure every single glitch is fixed by the end of the warranty. (Same with a car) The dealer will probably not outright resist that (he makes money on warranty work) but the factory will hate you. The end result is likely to be a destruction of your relationship with the dealer and the factory. Most Socata owners I know of who bought new experienced this. I wrote it up here. It may be different on a TBM (TBM owners get the red carpet rolled out) but I doubt it. There are various gotchas here e.g. the subtle difference between a mandatory SB and a normal SB (only the former is done under warranty) 
A twenty year old plane being described as ‘old’ makes me smile, although if they are flown every day by a school or some-such that can surely be the case. Mine are coming up on 70 years old and 44 years old, with about 3000 airframe hours combined. The 70 year plane has been flown coast to coast (and back) across the US, twice in the last twenty years, but in spite of that it is less capable of cross country flight… It is however much easier to maintain than the 1000 hr ‘new’ 44 year old plane.
I have enough personal vehicles (planes and otherwise) to keep running that it seems like every minute I spend on them matters. Complex modern systems would mean I’d have to sell most of them to maintain those remaining, and that’s not what I personally want to do. It’s a very different situation than running a charter business or flying a single complex plane for transportation. Equally fun though
On depreciation:
People who do buy new are often those who change every few years trying to avoid the worst of that. If you must have the latest and newest and if you have the cash to do that, that is fine as it frees a lot of airframes for those of us who don’t.
If you want to avoid depreciation as much as possible in net money terms, my experience has been to go for airframes (or cars for that matter) who have reached about 10-20% of their new price. In terms of airframes depreciation actually will stop at some stage and then its price will go up and down depending on time state of TBO components and to some extent by equipment. If you play with the online calculators available for certain makes, the oscillation of the value will be in the 20k range depending on the TBO items but oscillate around a medium value.
On maintenance issues:
Different airframes will be very different in that department. When I was buying 6 years ago this was one of the major items I researched as much as I could. As it is, I am pretty happy with my choice, I get to fly a retracable (whose gear mechanism is basically maintenance neutral) reasonably fast traveller which has few complex systems and no re-occurring AD’s safe for a prop hub inspection. The same goes for many of the smaller SEP’s. But throw in hydraulic systems, electro-hydraulic landig gears, complex control systems and/or airframes which are not reasonably corrosion proof you can end up in a massive black hole.
The 2nd factor is parts. There are some companies who are not exactly good at that. I heard pretty rough stuff of Piper recently and one Robin owner tells me he was grounded for over 6 months locating a particular part (HR100). There were rumours about Socata as well. Beech has a reputation for being quite expensive in the parts department. Thankfully, any part we ordered from Mooney in the last several years got promptly delivered within less than a week after ordering.
On high performance airplanes:
Both Turboprops and even small jets have reached rock bottom values at times. It is amazing to see ads and actually hear from people who bought what had been million $$ airplanes pre 2008 for 200k or less. One guy I encountered in the US started out shopping for a new SR22T as a personal transport, found the depreciation harrowing and went looking and gave up on finding a well kept and up to date Cheyenne and opted for a very low priced Citation 501 with LR tanks, fresh hot section and all phases current, which he managed to get for less than a used 5-6 year SR22! Given that his budget had been in the vicinity of that new SR22T, he now has about half a million he can spend on flying his 501SP additionally to his normal flying budget, which, I understand, he does quite happily. As an ATPL with plenty of jet experience he also had no problem getting the endorsement.
I thinkt the most important tip i heard when shopping for my own plane was that thinking out of the box can be very rewarding indeed. Instead of keeping your eyes on just that one plane you’ve been window shopping for, you can get a maybe much better deal by looking in places you never considered. When I bought my aircraft, I had spent almost 3 years looking in the wrong segment of a simple fixed gear/prop cheapo plane until I stumbled upon something I had never even considered because I tought it out of my reach. Well, things go crazy at times. A good friend just took home an Arrow III, actual IFR (not just “equipped”), EFIS, good time state and well preserved for less than 30k €. The same can happen if you look at the TBM market and might find that for your budget you can find something you never even considered for 25% of your budget and spend maybe anothr 25% upgrading and the other 50% flying it. I’d say with the budget and flying cost of a TBM, it may well be possible to fly a Cheyenne class airplane.
If you maintained it properly, hangared it, and ACF50-proofed it, it should be fine at the 20 year point.
Doesn’t your 21st Century TB20 have full 2-part epoxy coating throughout ?
Just about all the spam-cans built since the 90’s have the full epoxied airframes and this, IMHO, goes along ways in preserving them, vs nothing or the old chromate stuff. It also precludes having to use ACF50 or any other preservation oil.
Not epoxy… Socata use 2-pack Mapaero paints. Does anybody use epoxy for the topcoat? I don’t think so. It has little UV resistance and AFAIK would need a UV lacquer on top. Epoxy primer, yes.
Socata paint is generally good but they had some “accidents” where it peeled off in big strips. Mine did that around the filler caps. Internally, mine is as new, zero corrosion.
Zinc chromate is only good for “WW2 authenticity”. It is a poor primer.
I don’t think epoxy primer precludes ACF50 – does it?
BTW the Socata parts rumour seems to have been purely a rumour. I have never actually seen a real example. What does happen quite often is that a maintenance company has not paid the supplier and got their credit account terminated, and they don’t have the cash to send up front, and they tell the client that they cannot get the parts for X months
However I think that if you rolled over a TB20GT and broke the composite roof beyond repair, you might have a problem in that if they don’t have any, they probably won’t make any. I don’t know about cowlings, but they can be repaired / copied.
Any kind of primer, chromate included, is a lot better than no primer. I have a spare set of rag wings, zinc chromated at the factory about 70 years ago and they look OK. If I ever re-do them I’ll strip and reprime them but it will kill me to remove all the little original QC stamps etc…
I was careful when buying my planes to get ones that had been internally primed. The 70 year old plane has a factory chromated wing structure that still looks perfect, plus a modern epoxy primed internal fuselage done in the 90s. The 44 year old plane was fully internally primed at the factory with what looks like the same old light green epoxy primer that I remember from the early 80s. DP40?
Comanches were famously zinc chromated at the factory and a friend’s looks absolutely perfect after 56 years, mostly flown and hangared in Kansas (for the first 40 years). Actually he just sold it and the buyer was as blown away as I’ve always been looking inside the structures. They look like they were painted yesterday.
