Heh, yeah, we need a new car model, I think.
For art stuffs, you can snag some foliage/terrain/road art from the pacific art package, and for stuff like signs or maybe even cars, you could look at opengameart.org. They tend to have a lot of public domain/cc0 art stuffs up for use, may be worth a look there.
For basic buildings, you could also use the convex editor tool to block stuff out.
As for using github, this may be useful info, if not lemme know what spots you're rough on with it: http://wiki.torque3d.org/git:how-to-use-git-with-torque