While I can't answer your question relative to the 12 volt grinders, I can tell you what I did.
I bought one on these grinders for $39 http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_36988_36988
I then got a AC drive motor with a gear reducer (one piece unit) on e-bay for about $60. You need one with about 1/4 HP motor being reduced to about 75 RPM out of the gear drive.
I made a pressure treated mounting plate to hold the two on and to get the shafts aligned. Then just had to make a coupling to connect between the gear drive shaft and the shaft the hand crank is on.
I don't take this unit on the boat and I grind all chum at the dock.
This thing will eat herring as fast as you can feed it and it won't jam up.
When grinding big mac's, you need to feed one at a time or it will jam. The same with big pogies. Small macs/pogies are no problem.
I built a second one for my buddy with a 1/3 HP motor and a 66 RPM gear reducer. This one won't jam with big macs or pogies.