Hi,
i would like to build something similar, basically an autonomus / rc platform built around arduino.
Was considering using an Heng Long tank as chassis / motors / gears. Seems a decent enough platform for mildly rough terrain.
Plus, it would look cool!

My question is: do you really have to power your engines through the micro? why? that seems to me the work of the esc. From the arduino board you send the signal to drive the motors, not all the amps to power them.

See for instance this diagram:

(he uses servos in his case)

taken from:
http://www.irit.fr/~Nicolas.Lassabe/robot.php

This, of course, if your really want to work on micro programming and have an autonomous robot rather than just repair your pershing.

Regards