I did some scouting at local pawn shops for suitable power tools w/ strong motors over the last two days. I ended up finding 2 14.4V Craftsman drills (each of which came with 2 batteries and a charger) and a 14.4 V Black and Decker circular saw (w/ 1 battery). Grand Total $140...Doh!
I shopped around with the intention of matching the two drill motors so that they would be balanced in torque/current/dimensions/etc. I made the assumption that being the same voltage and brand would be enough to get evenly matched motors. Unfortunately when I disassembled them later I found that the Larger (red) one had a motor much larger than the other (gray), and the transmission was very different... The circular saw has a motor similar to the larger drill. All three have different drive gears.
So the plan at this point is to ditch the transmission from the larger motor and the gear box of the circular saw and find gearing for these two to make them drive motors. Next time I think I'll just buy identical motors...hopefully these last a while. I've been scouting out all the other things I'm planning to use as well: steel base, wheels, axels, bearings, etc. I'm also working on a custom motor driver circuit which hopefully will be cheaper than buying a ready made one...yay shopping! I used a uC and the power MOSFET from one of the drills to rig a test circuit before I order the parts for the driver. Tests pretty much confirmed that I'm on the right track, but I'm going to formally design it to try and minimize screw ups. During the test the motor had enough torque to throw itself off the desk...I can't wait to get this thing on wheels :)