A Funke (TRT800H or earlier model) with remote head was chosen by my ex-club because it was cheaper and smaller. The downside is a fiddly interface due to the small knob and buttons.
A used KT76A or C would be very cheap, and if/when it stops working you can slide in a TT31.
Personally, I’ve just had a used GTX330 installed which I thought the best compromise.
Here is a vendor website on the TT31 which notes issues with ‘plug and play’ installation into a King tray, although in relation to ADS-B functionality not at the same level of detail as noted above.
I installed a Garmin GTX335 that was available at the time (2019) with both a Garmin promotional price and an Aircraft Spruce discount (a AS vendor friend and I bought a couple of them through his business). Otherwise the choice was mainly driven by the 335 being available with on-board GPS and a plug in encoder – I wanted as little hardware and wiring as possible. Otherwise I would’ve gone in almost any direction other than Garmin, and would likely go in a different direction if doing it again today. Garmin is a horrible company, remarkable arrogance and customer manipulation are their core competencies 
Silvaire wrote:
is a vendor website on the TT31 which notes issues with ‘plug and play’ installation into a King tray, although in relation to ADS-B functionality not at the same level of detail as noted above.
It is plug and play for the KT76X as long as you do not need additional functionality. At the time it came out, the requirement in Europe was and still is Mode S, not ADS-B. So the TT31 was designed to do that: Replace a very popular Mode C Transponder with a slide in replacement for Mode S. This it does.
The moment you want ADS-B, you need to add a connection to the WAAS GPS Source. this still can be done using the KT76x rack, at least for ADSB-OUT. It is a comparatively small connection work which any qualified shop can do quite expeditiously.
The way I did it was that I initially installed it in the KT76 rack I had and the avionic shop wired the connection to the GNS430 into it. When we did the big upgrade, the avionic shop used a new Trigg tray and rewired the connection to the GNS430W, as they had the whole panel apart anyway and a tray available. They told me that the moment you want additional functions to pure Mode S, you might as well change the rack as they have to take it out anyway. As long as you want a simple exchange of a Mode C to a Mode S transponder and have no WAAS source to be wired, you can leave the old rack no problem.
Silvaire wrote:
Here is a vendor website on the TT31 which notes issues with ‘plug and play’ installation into a King tray, although in relation to ADS-B functionality not at the same level of detail as noted above.
I can’t find anything on that page which implies that you must have a Trig tray for ADS-B out functionality. Indeed in several places it states the opposite.
Cutting and pasting from the end of the first paragraph (‘Overview’) of the linked Aircraft Spruce webpage I provided in Post #12:
“It provides pilots with class leading Mode S technology that is also 1090ES ADS-B Out capable too (ADS-B functionality requires a Trig tray).”
The Trig needs a GPS source for ADS-B out. There is a Trig device which does this, approved in the UK. (Only?)
Silvaire wrote:
Cutting and pasting from the end of the first paragraph (‘Overview’) of the linked Aircraft Spruce webpage I provided in Post #12:“It provides pilots with class leading Mode S technology that is also 1090ES ADS-B Out capable too (ADS-B functionality requires a Trig tray).”
Well, that’s incorrect! Aircraft Spruce should have read the Installation Manual properly. (What they write in the Q&A section seems correct, though.)
As I wrote in post #7 my club has an aircraft with a TT31 in a King tray which does ADS-B out just fine. Mooney_driver wrote the same in #9 and #13. But maybe first-hand experience by aircraft owners is less trustworthy than what a distributor writes in their marketing material?
As I wrote above, it is correct that if you want to replace your old parallel encoder with a serial one when you change from the KT76x to the TT31 then you do need the Trig tray to have ADS-B out.
But maybe first-hand experience by aircraft owners is less trustworthy than what a distributor writes in their marketing material?
Well, at least you read the link this time. 
Distributors don’t usually add discouraging info to their catalog info for no reason. In this case it seems they know half the story, missing that some customers with some encoders can reuse the existing tray while adding ADS-B functionality, while others cannot. With that in mind it might also be worth re-reading my post one more time to note what I said in relation to your comments on the subject of ADS-B functionality (bold added by me this time):
Here is a vendor website on the TT31 which notes issues with ‘plug and play’ installation into a King tray, although in relation to ADS-B functionality not at the same level of detail as noted above
I think when installing anything that is intended to be backwards compatible, the guy spending his money is well advised to read everything, everywhere that’s available from the field because the manufacturer likely did not address every possible pre-existing situation or installation detail in their design. Then having done that you’ve minimized your level of risk and are in the best position to decide whether to spend your money and give it a try.
Outside of the issues with ADS-B, customer comments in the Aircraft Spruce link also raise an issue with incompatible tray pin size in one particular customer’s installation, requiring extra labor to install the Trig in the King tray. That doesn’t seem to be common but I’d guess it’ll be an issue again for somebody, somewhere.
Silvaire wrote:
Well, at least you read the link this time.
I actually did read it the first time but missed that part, concentrating on the Q&A.
Distributors don’t usually add discouraging info to their catalog info for no reason. In this case it seems they know half the story, missing that some customers with some encoders can reuse the existing tray while adding ADS-B functionality, while others cannot.
Every customer can reuse the existing tray and get ADS-B functionality. The KT76 doesn’t use a serial encoder so the customer must have had a parallel encoder. Only if the customer also throws out the existing parallel encoder to install a serial encoder do they need the Trig tray.
Situations in actuality do vary, which is why you need as much info as you can get before committing to any particular solution. Some people could install the Trig as a ‘drop in’ to meet their needs. In my aircraft’s case, my encoder was in poor shape and the KT76 was also moving out of calibration. I’m sure that’s very common with a system installed decades ago and understanding the limits of the Trig unit in a King tray is important in that circumstance. Anybody in a similar situation would likely choose to install a new tray regardless of the transponder they’d choose.
Given the issues with all my hardware, I decided to replace the whole system (transponder, plug-in encoder and adding a GPS receiver via the GTX335) versus going with a u-Avionix Skybeacon in combination with the existing transponder, which was my initial plan to transmit ADS-B. I have no need for a Mode S transponder so that was not a consideration in my circumstance. Clearly I would not have installed a Trig unit, a GPS receiver and a different and non-optimum encoder just so I could reuse the tray.
