They all come stock but can be highly modified. There are a few exceptions (prize cars) that are given to you modded and cannot be changed. Another point of interest is that if you do mod a car, it warns you if that mod is going to put it in a different class. If you do mod a car to the ritz (up to the Works package but not getting that) everything can be uninstalled and sold. So let's say you have 200k worth of mods dumped into an Rx7 and you find out you dont like the way it drives and it really is a POS, you can sell all of the mods, get your 200k back. If you do the Works conversion, that cannot be uninstalled or sold.
