My choice would be to put a hub motor on the rear of that thing. Simple, cheap, and easy. the 3-speed will be redundant with a motor (heck, I built a single speed as I just don't shift). 250 watts is about the power of a strong rec cyclist, so if you like to pedal no need to go much above that (i.e. 36v motor at 10amps). If you go a lot more, the motor will overpower you.
Originally Posted by
2old
One thing to remember as you search: the rule of thumb for hub drives is you should (must) ascend at half the top speed of your system (probably 9 or 10 mph) or too much energy is being converted to heat and you could fry your motor.
True that!
In my experience, the controller will go first (well, they aren't that expensive). It will probably take 5-10 minutes to overheat - but of course that depends on how slow/heavy the load is.