bolt wheel to servo.x2
bolt two servos to chassis
bolt castor to chassis.
bolt a bumper to chassis.
wire up some sensors.
wire up to micro.
what do you need a transmission for? whats it using as a power source? a V8?
direct mount the wheels to a servo, or if you need stupid speed/torque/power, buy a motor with gearbox attached. it would be cheeper than building your own.
serriously, make little cheep simple stupid robot. when you can control cheep simple stuff, then you might manage to not trash expensive stuff too often. said by someone who thinks battery fires and burnt out micros are just minor issues.