Well a descent example would fetch about £6k - £10k or more depending on trim and if Auto or manual, UK or JSpec and level of mods.
I personaly would buy a Stock Supra TT and then do what you want to it (thats if you want to) so you know its done right, there are alot of low quality parts out there for the supra, and would hate to see you buy one and it fail a couple of months down the line, if you do buy one with mods then just run the mods past the people on here and they will say whether to steer clear etc.
My Wife has a MR2 Turbo and whilst it is a great car, very chuckable and really fun to drive, the supra is so so much better in all areas IMO and sounds better too, but then the supra is a bigger car and is a tad thirsty (ok pretty thirsty) on the spirited runs.