Check out the Nanotubes from AndyMark. They come in 20" and 31" lengths and can be easily assembled into a 4WD, or 6WD drop center configuration. Just cut and mount some crossmembers, a belly pan, motors and wheels, and you're (mechanically) good to go. We've used them 2 of our 3 years, and have been happy with them. They're also easily reconfigurable for a demo or practice bot.
http://www.andymark.com/product-p/am-0949.htm