Rear seats are hopeless for anything other than very occasional use. Zero leg room unless the driver and passenger are midgets.
They don't really rust. The galvanised body is great in that respect. Some rusting on rear hatches has been reported though, not a common problem really.
Yes, they're very tail happy in the wet. You can easily control it with some care and experience - or RLTC.
19 MPG is about right. 29 MPG is only realistic if you only do downhill journeys.
Imports are dearer to insure but not all that much more if you phone around. NA insurance is usually about the same as the import TT.
Spares are not a problem. The option extra side skirts and spats are the only item I know of that you can't get new any more.
Dunno about your local meets.
The M3 is a good car too. If you can find a good hard top one it would make a good alternative to a TT Supra. Personally I wouldn't be seen dead in a convertible one though (or an aerotop Supra)
They get done up like dog's dinners because people have no taste.
The manual TT is the best choice for most folks but the autos are very good too. The manuals are harder to find and dearer. Test drive both and see if you like the auto, they're surprisingly good.
How much would you need to pay for a standard/light modded TT? How long is a piece of string? A 1993 auto TT with 90,000 miles and a Bomex/Veilside bumper, ironing board spoiler and loud dump valve will be about £4k, a 2002 6sp VVTi with 40k miles and OE bodykit will still set you back 18-20k, if you can find one.
HTH