I put a fair amount of research into the robot platform. A standard approach is to modify an R/C car, but I wanted something that could maneuver in tight spaces, be durable, and capable of climbing over modest obstacles.
I selected the Dagu Rover 5 since it appears to be pretty capable. The only negative I've heard is that it can occasionally shed tracks if heavily loaded, but the videos I saw of it in action look perfect. It can also be changed to wheels later, if you prefer, so its pretty flexible. It includes shaft encoders on all 4 motors, which can tell a microprocessor how much the shaft is turning. This would be very useful if I wanted to make the rover autonomous.
Driving the motors was an interesting challenge - each is rated at 2.5 amps stall current, which is a bit over the rating of the standard LM293 controller boards. Dagu sells a $27 board specifically for this rover, with 4x 2.5 amp outputs, and circuitry to read the shaft encoders. I eventually determined that's a pretty good deal and selected it.
Here's a demo video a fellow on Youtube shot of his Rover 5 buzzing around outside.