Can you provide a simple sketch of the robot you plan to build? Or at least some detail on what sort of robot? There are so many different kinds of robots, how can you expect any useful suggestions?
I just got a new $25 R/C toy, just for the purpose of taking apart, and turning it into a walking robot. It's an Nsect Nano, a walking bug. Two motors, six legs, and all the gears and crap already done for me. Should be easy to stuff an AVR, H-bridge, and IR sensors. Thing is fast on a flat surface, not bad on carpet.
Just curious... How can you do the electronics first, then design and build the mechanical. I think I smell smoke...