I searched high and low for a good, reliable drivetrain solution and settled on an electric wheelchair due to the reliability. I combed Craigslist
and scored a used one for $75. (these things cost upward of $1,000 if new!)
Strip the chair to its frame as it will most likely need to be repainted or thoroughly cleaned. My chair had an extra set of wheels out front that I stripped for aesthetic reasons. Make sure you keep all the wiring harnesses as we will reuse those! Plan on buying a new battery if the chair has been sitting. (these run about $75) I purchased a 12 volt, 35 amp hour battery which is sufficient.
Once stripped, the frame needs to be degreased. I stripped and repainted the wheel rims at this point as well.
You will need three solid points to mount the main structure of the robot to the base. I used some parts off the discarded frame and had them welded to the front to make two of the mount points. You can see three 1/4 inch holes in the frame where the top of the bot will mount. At this point, test fit the battery and add any mounts or tiedowns necessary to keep it from moving around.
Once you have the frame reconfigured, repaint with a black semigloss spray. I used Krylon. The wheel covers on the back were also sprayed with the bedliner.
When dry, reassemble the frame and set aside.
Note - don't worry about trying to make the frame lighter. Any additional weight on the bottom only makes the robot more stable when moving.