EDIT: I read 'character' as 'car'. Why the heck did I do that for?! 
Here's one way of doing it:
Spawn a prefab of a generic car, or all the cars and everything packed into one prefab. Have it under nested GameObjects, like:
DragCar
-- V12 SuperCharger
---- Body
---- Decals
-- V8 Turbo Diesel
---- Body
---- Decals
<etc>
Have a script attached to it that checks what the player settings are (ie. decals, Nitro, exhaust, paint job, etc) and enable/disable the child game objects depending on what the car has.
You might get a minor performance hit but since the script will kick in ASAP, the player shouldn't notice a big black box thing as the car is customized.