well EM3ev is going to sell you much, much higher quality stuff than the other china/Hong Kong retailers. Paul really does a good job. Last I looked, he didn't have a cassette compatible motor.

Bafang CST is a motor you would like (if you need the Cassette). That style motor can be built with either 36/ 48v 500 or 1000 watt, the only difference being the controller (which pushes more amps through the motor providing more power (and in the case of 36 vs 48 volts, they will have different winding to keep the motor from going too fast or two slow)). That Bafang CST motor is about $270 + shipping (which is going to add at least $100 to the cost).