Radio Shack may have a winner
Vex Robotics has some very sturdy-looking components such as these gears and sprockets, and especially these large-scale treads. Robot magazine will debut with an article about using Vex components to build a scaled-down iRobot PackBot - see it in action here, climbing over some concrete blocks. I had the chance to see Vex up close at Robonexus, and was impressed with the quality and reasonable price.
Forget about the radio remote controls, add a real brain and some more sensors to these parts and you could have something large enough to climb stairs and rugged enough to last for a while. Prices are reasonable, and you can order online or pick them up at your local Radio Shack.