I'm looking to trade my SSM II drives and put TRS on it and was wondering what specific things to look for when inspectingused or rebuilt outdrives.

not too sure but that's why I brought my 2 spare drives to my merc dealer to have them check when I bought them!they were OK so I had nothing to change... turned out I sold my boat last year and never had to use the spare drives! if ever you need them let me know! fresh paint !

Externally the one thing you can see is the thickness of the housing behind the prop where the carrier retaining nut goes. The TRS housing is on the thin side to begin with and has a tendency to corrode in that area.

Pull the drain plug and see what the oil looks like.

Look at the prop shaft for twist, look where the taper on the prop shaft is - see if it is galled. Turn the drive, see if the prop shaft is bent.

Grab the input shaft, see how the u joints feel.

After that I don't think there is much you can see without dis-assemble & magnaflux.