The thing about using an inexpensive voltage reducer is that house voltage is not always consistent. It's not gonna be great for fine-tuning on the low end, but you could cut the rpms by 1/3 to 1/4 and try it from there. The main concern is to have lower speeds for safety.
Torque-wise, it kinda depends on your setup and how much continuous grain is being fed through. The crusher I'm using is built with skateboard bearings and a light feed and so it's not much of a concern. Personally, I'd go with a cheap drill. It'd take the fun out of the project but would be easier to design and adjust.