Your design looks good.
The caster wheel on the back will do just fine. The differential drive feature you've added will allow your robot to turn on a dime.
The front of your robot seems to be large and roomy (yea, I just made up that word) which is perfect for mounting various sensors. For sensors you could possibly use Photoresistors (from the $50 robot) to measure light, an IR rangefinder to measure distance and/or track objects, Sonar (same as IR rangefinder), touch sensor, flex sensor, some sort of device for capturing video such as the CMUcam, accelerometer, etc. There are many possibilities. For the motors you will need either two h-bridges (per motor) or a motor driver. If you use servos you can plug them directly into the Axon.
