people here are harsh.
The factory TRD
kit is only the body pieces. It would seem that your car came with it... but it could have also been an aftermarket TRD
kit. Doesn't really matter.
Some dealers sell pre-modded Celi's with TRD
parts (exhaust, springs, ect). Post a picture of your exhaust and we'll tell you if it's stock. There is no question that your suspension is stock.
First mods should be TRD
exhaust and AEM Cold Air Intake
. After that, you could get headers from any of the companies who make them for the Celica. Don't expect huge gains... maybe 10 HP from all three combined.
If you're serious about a turbo, search around the site. There are MANY threads, and MANY aspects you need to consider, from price to reliability. Too much to cover here. Read up and if you have a specific question, post it up.
BTW, I'm assuming your car is stick, right?