You'll be hard pressed to find a used motor that is the exact torque and rpm that you want (which is over 50 in-lb and 175 RPM). So, most people get garage door motors. Mine is 1725 RPM and 1/3 hp. It is slowed down 10:1 with a gear reducer. That gives me 172 RPM and multiplies my torque to 100 in-lb. I have a lovejoy connector in between the drive shaft of the mill and the gearbox shaft.
The main question is- do you want to slow it down with a pulley system or a gearbox. I wanted to stay away from pulleys. Pulleys are cheaper than a gearbox.
Sorry if I oversimplified. Here are previous posts to check out (albeit I don't think they make this gearbox anymore):
https://www.homebrewtalk.com/f51/motor-size-monster-mill-195335/
and
https://www.homebrewtalk.com/f51/looking-grain-mill-motor-advice-246959/#post2944041
with a vid w/ it in use (about in the middle of the vid).