Because of the lack of torque I would probably stick with some lightweight 17's. A good rule of thumb is 1lb for every inch in diameter, which means wheels that weigh around 17lbs or less would be ideal. Lighter wheels are considerably more expensive though, if you don't care much about performance, then you can go heavier.
The bolt pattern for the Celica is 5x100 and the stock offset is 38, try to stay close to the stock offset to avoid fitment problems.
Use this:
http://www.1010tires.com/TireSizeCalculator.asp to find the closest overall diameter to your stock wheels. I *think* stock size for a GTS is 205/55R16, in which case 205/50R17 seems pretty close. 215/45R17 would also be pretty close, but the tire would be slightly wider, giving a slightly more aggressive look and a little more traction in ideal conditions.