You can drive a motor with a variable freq drive to very low RPM's, the problem is cooling and if you put a "stinger" (small dedicated fan motor) on it, they survive well. I had such an application on a conveyor and after the second motor Baldor sent me one rigged as above. It may still be working.
Horsepower would be tricky. There are horsepower and torque curves available for most modern and you could probably interpolate. But is it that critical? Does the saw have a power feed and does it under a load?