If you're using an AC motor, the good AC conversions I've seen have run at 300 volts or so, using 12v batteries. People have run these motors at 120 volts, but are not happy with the acceleration. (Update: See the HPEVS AC-50 motor. It works fine at 120 volts.)
DC conversions seem to have variable voltage, depending on how fast people want to go, how heavy the donor car is, what kind of bone structure it's got to support battery weight, and how much money they've got to spend on batteries. 120 volts will give you a pretty snappy car.
When you ask about RPMs, are you asking about torque, does more voltage equal more torque? If so, then the answer is yes, pretty much.
A generator will provide the quantity of power you need to run your electric motor, but not the rate of power you'd want for acceleration. So keep your batteries, and use your generator as a charger.
You don't have that many choices, really. If you're going to use a 3-phase AC induction motor for your conversion, the best option is a traction motor, one that's used to move something. The best ones for cars seem to be coming out of old manufactured electric cars, like the Ranger EV, for example. That one's made by Siemens, I believe.
Other choices include series wound DC motors, and brushless DC motors like the Rav-4 EV's motor.