The vfd's mentioned above are what I am in the market for, but I made a score on a 2hp 3-phase motor for cheap.

The treadmill's have been used by many. I think they usually start with a 3/4 hp motor & go up, so you should be ok since you don't need a lot of torque at the slow speeds as you wouldn't be adding a lot of pressure at the slow speeds anyways...due to the heat. Secondly you can help it along with a small drive wheel and larger secondary wheel to keep the speed down and the torque up.