Quick Update: The Big Trak's superior size, zero turning radius and general aesthetic appeal won out, so it's new test platform.
I posted a few pictures of it as it is currently. Although the original circuitry lends itself to modification, it was all removed (except for the shaft encoder and drive motors) to facilitate testing various sensors and other electronics. Likewise, the finished version will reside within the original plastic body, but for now it's been removed for testing.
Right now it's 16F877 powered (soon to have multiple PIC's from the way things are looking) is equipped with HPWM driven IR detection, an L298 full bridge driver circuit with diode protection and a linear adjustable voltage supply with nicads for power.
At the moment it is programmed for simple obstacle avoidance and pet irritation, but compass and sonar experiments will follow shortly.
BTW, to anyone who's contemplating a small H-Bridge project, IMO, the L298 is the way to go. It's got built in current sensing and it doesn't seem to have the hyper-sensitive, freaky little "noise" issues that plagued the mosfet drivers I tested.
It's cheap too, you can pick them up online from Mouser.com