first - export obj with separated parts,
Option 1: import to 3DC to sculpt room and then switch to paint room, you will be able to paint on vertex and use smart materials etc.
Option 2: import for (Mircro vertex) from what I understand it will auto unwrap for better performances (no need to change anything, it is automated) and you will be able to paint
But I don't see any reason to not unwrap it (fast and dirty auto unwrap) you will have more flexibility. If you want only car paint shader, like paint car, then there is no need to use 3DC, just apply it in blender etc and render