Stepper motor for ebike
Stepper motors have very poor maximum RPMs and dreadful torque at high rpm. Also, controlling stepper motors requires a special controller and it's hard to find high amperage controllers. High-amperage stepper motors are also very expensive.

There's really no advantage to using a stepper motor. You don't need its precision (you can tell a stepper to rotate CW 145 degrees then back CCW 32 degrees, but why would you need that in an e-bike). 

Go with an appropriate brushless motor.