Your aircraft would have the ball type, those were pretty standard for many years when all we had to contend with was mode A and C. Mode S and ADSB and all the other data that is sent out over that short burst from the transponder changed things, I would not rely on the ball type because to me the word " sometimes " leaves a lot to be desired. The GDL 82 can probably use the existing ball antenna but if it were me I would go with the other style if there was any doubt.

As far as who can install what, some of it is a bit of semantics. What the dealer has that only dealer access has is the installation tool for doing software and configuration - especially for remote units. For panel mounted units they can be configured right from the screen of the transponder. Depending on what kind of encoder that is used, some use the traditional remote ones and that's fine, I have used those as well as the Garmin one that is mounted right on the unit, either way works the same.

After the installation, any repair station or even an FBO if so equipped and with the right credential ( someone who is certified to perform the transponder cert ) can test the system and ensure its working correctly.

Long thread here to be sure, but there is a lot to consider when doing this stuff and every day we creep closer to Jan 1 2020.

Thanks again

Mike Sutton
Cubcrafters Avionics