You should have an avionics master to disconnect the avionics during engine start, because the starter motor puts a lot of crap onto the positive supply line.
The Collins product was never marketed.
I would not have thought that the Americans would have the UK-style "civil liberties" issues with Mode S. After all, unless you fly in the "wild west" and under the radar, you will be tracked everywhere you go.
I can see Narco did not bother to do a Mode S transponder because Garmin owned the avionics business by then. The only openings were for "cheap" Mode S transponders but (hey what a suprise; you'd never think this is aviation would you) all those which came out were (basically) at Garmin prices minus 0.0001%
It only costs about £100 to make a Mode S transponder, in 1k batches, but nobody is going to bomb the price and anyway most of the installs are done by avionics shops and they will always sell the product with the biggest trade discount.