go with a variable speed motor. i think you can still get them at lowes. you can sharpen just about any knife out there with the wheels. convex edge knives have to be done on a belt sander but they can be finished off on the slotted wheel.
you want the wheel to be 2" larger than the motor housing so your fingers clear the housing.
you want the wheel to be 2" larger than the motor housing so your fingers clear the housing.