You can have the gears micro-polished and cryo-treated. Otherwise, go with as new a model as you can to get all of the factory improvements (944 S2 would be a good choice).
Transaxle - General Information, Codes, and Gear Ratios
Stay away from the turbo transaxles as they will be geared completely wrong for your car and will suck the life out of it (unless you're planning on an engine swap or adding power to your engine)
The transaxles are Audi-based units and some parts cross over to the Audi 5000 units, so that might help in your parts search.