Hi there,
Perhaps it'll be best to disassemble/reuse a ready-made model.
for USD$8.75, you can pick up two units of this "line following robot". They're normally 8.75 per unit, but with the promotion code below you get two for the same price.
**broken link removed**
add two to cart, use promotion code "GETONEMORE" in cart for discount to be reflected.
While it does require some monetary sacrifice, it just might save you a fair bit of time.
It's also not exactly in the spirit of "sharing", but just might fall under "reverse assembly". Plus, you get the motors, gearings, wheels.. just redesign and rebuild (or repackage!)
disclaimer : I am not promoting, or affiliated with axshop in any way. Just was browsing yesterday and thought that you might find this useful.
pps : shipping takes 1-2 weeks, but it's free ship.