[PAR_WIG1350]

Regarding variable power supply control.
This would provide a rather simple solution. I see this working well with a dpdt switch for forward/backward/center off with a throttle rheostat and a potentiometer for steering. The rheostat would be placed between the power supply and the pot. The output from the rheostat would connect to the center lead of the pot (wiper) and the two motors would be connected to opposite unused leads (the side leads). the dpdt switch would be connected to both battery leads in a H-bridge-like configuration (with out the break to power or break to ground options).