Hey,
Sounds like a wild project. For fear of life and limb, I like the idea of starting by hopping up a golf cart and then perhaps do servo controlled steering too? wait... that's much more dangerous!
For what its worth, I've put a PID control application on the Playground that could be a good starting place. It's setup for temp control right now, but easily adapted.
http://arduino.cc/playground/Main/BarebonesPIDForEspresso
tim