Crankandstein post suggestions on how to do it - see the links they share near the top of this webpage - but I don't think there are any "specs" per se.
Torque - fairly high, so you don't have to feed in your grain a little at a time.
Mine is a right-angle, 62 rpm (slow. you can go up to 500). don't remember the torque. 1/17th HP.
Probably slow on the RPM and overkill on the torque - when you gear down the speed, you gear up the torque. I'd toss in a rock to see if it would stop it, but I don't want to tear up my mill.
all that said, find an inexpensive motor. Build it!