Quote: "if a do a 3D model for each possible mix, my game will be so big"
That's what you'll have to do, however not for each car combination.
Each body panel of each of your cars needs to be a separate object. Each aftermarket part also needs to be a separate object.
you'll then need to create a special loading routine for your car to load the base object (the body) then add to it the bumpers, bonnet, doors, boot, side skirts, wheels, tyres and spoilers. If your smart, you can even joint them to the car body so they can fall off when hit with a certain velocity.
Your signature has been erased by a mod because it is way too awesome!