As long as the body kit goes with the styling of the car then it looks good. Like the buddy club body kit for del sol's look really good, same with teg's. A horrible body kit is the invader and eurabuni shogun style on the 2nd gen dsm's. Those look like trash. The newer mustangs look good with rousch kits or cobra kits, but if you have a cobra kit on your car and its not a cobra, make sure you shave the emblems to where it in no ways says its a cobra, that is rice. Type R badges on a non Type R car is also rice.
For your celica, if its an auto you should def swap in a manual. If its already a 5 speed, maybe go turbo or get nitrous. Turbo's are fun, nitrous is cheap and easy to instal but read up on both because they can both blow your engine to peices if used improperly. As for the convertable... props man, now make it look good, go fast, and chicks will be all over you.