Not to discourage you .. but your mesh(es) contain allot of double / hidden / unnecessary polygons. So it would take quite some time to clean them up before they can be considered game-ready and / or somebody can continue with them. Obviously, SketchUp is ( at least partially ) to blame ;)
Check out the following video to see some clean-up work on your mesh, so you can see a bit of the work involved.
http://youtu.be/eqm94mVtI4k