you must get 18's.
you will regret later on if you go for 17's.
GT is not going to be fast anyway if it is not Turboed.
Plus Celicas were not meant to be drag racing cars.
18's give it a NICENICENICE LOOK!
Ehh... sure, the 18's may look nice, but you'll lose Performance.
If you plan on autocrossing or road racing at all, go with 17"s. If you don't? Well, go with the 18"s. I'm just telling you, proven that 17"s outperform 18"s. (race cars that ride on 20" rims don't count, their suspension is tuned for the little sidewall