The tail end of the kn5 is all the 3d models and material definitions, I haven't looked into how they're structured yet so I can't really build new kn5s with any rate of success. I'm just planning to wait for Kunos's modding tools really, it's silly to reverse engineer the file formats when they're planning to hand out tools to do the same thing. Textures happen to be fairly straightforward.
Same goes for data.acd - I know enough about it to pull files out of some of them, and tweak cars a bit, but it's really not user friendly enough to release, and anyway I imagine Kunos is planning to hand out something for editing them.