I have also assisted with ADS-B installations involving the Trig TT-31 *and* the TT-21/22. Also the Garmin 330ES to a lesser extent.
A couple of points.
1. If you had a Garmin 330, upgrading to the 330ES is probably your cheapest route. That and your 430W and should be "good to go". <I know of a 330 that is available.>
2. If you had had a KT-76 that died, you could plug in the TT-31 and use it as a basic transponder until you were ready to install the TRIG tray for the ADS-B.
3. If you have space constraints, the TT-21/22 can be attached as a REMOTE device for some EFIS systems *OR* can be installed with a 2 1/4" control head otherwise.
My TT-22 is currently being fed by my Garmin 496 (I had the GPS OUT wire easily available).