Clarid, I think you can only do the body mods on the gt2 game. The gt3 game lets you do all the suspension, engine, tires, etc , but not the racing mods like in the gt2 game.
Body mods are the ones that let you choose which type of paint and graphic scheme you want for your car. Plus, it get you some stuff and adjustments you wouldnt get with just a bought and souped up car.
Its a cool feature on gt2 that is not on gt3 and it really missed.
Later.