The motor shield is for small DC motors, not for ones found in the wheelchair you mentioned in your original post. You will need a bigger speed controller to do that, like the victor or jaguar you also mentioned. The arduino by itself can generate the pwm signals to feed the speed controller of your choice. The
roboduino is an arduino compatible board that already has headers for pwm cables. With a normal arduino board, this is basically how it would need to be plugged in. A fuse would be a nice addition.
