My thinking on how to power my mill (an "ugly junk" Corona mill that is being driven by an old 1/2" drill at present, and which grinds my grain bill just fine, thank you very much) ran something like this:
1. The trouble with AC is that the motors spin too fast, unless you use a pulley/belt to reduce RPM, which can be hard on the bearings of your crusher, and which require a shroud be constructed for safety. Either that or another technical accessory, such as the Hz controller specified above. You may get lucky and find a cheap AC gearmotor that will do that job, but I'm not counting on finding one cheap, and I don't want to spend any more on a power unit than I did on my mill ($24 delivered).
The trouble with DC is all the various voltages out there (such as the 130V motor above requiring a special power supply to run it)........except one, the familiar 12VDC that powers all manner of accessories on our vehicles.
Any auto wrecking yard ought to have mass quantities of this stuff, and pretty cheap if you're not looking for anything special. The most obvious geared-down low RPM motor would be a wiper unit. There are also other things.....I saw a transfer case motor coupled to a grinder in a You Tube video, only about 100 rpm. Great!
Apart from an old 12V / 25A power supply I have around about as massive as two concrete blocks, I also have a deep-cycle 12V battery that does absolutely nothing but sit there as backup power for one of my sump pumps. If you think about it, you probably have some source of 12V power sitting around....one of those little booster box things to start your car probably packs enough juice to do a batch of grain, etc.