One other option Mark: get in touch with Roger Plant (http://www.bigcheeserobot.com/http://www.bigcheeserobot.com/) and see if he still sells the Curtis speed controllers. They were £50 each when I got mine (so £100 for both motors) and are easily powerful enough for wheelchair motors.
They take a 0-5V analogue input so youll still need an interface of some sort.
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