hi,
by reading your post i was thinking about this, maybe i want a car too in unreal. i do not know exactly how its working in unreal, but here is a post and it should work like in unity 3d. in unity i made my car out of ~ 60 parts and combined them on gamestart to one mesh. when a crash happens you could look where the collider (simple representation of the car out of boxes) is hit and then do damage at position or split mesh apart and reparent it to minimize draw calls.
this shows some examples
hope it helps 
cheers