Surplus center is a great place to buy, but is often sold out on proper motors; you might get lucky though.
The key is getting enough torque while maintaining a reasonable RPM. I'd say to try and find a motor with 20+ lb-inches of torque. Probably want at least 50 RPM's, but torque is more important.
They come in all sorts of these torque/rpm combos so just snoop around. It would be a killer setup for you though and worth the effort.
Check out grainger for a coupler setup to connect your gear motor and mill. Wire it with a 3-pole switch if you want to get fancy and add reverse
PS - there is a pic or YouTube video out there somewhere of a guy who has a gear motor connected to his mill. Everything is mounted to a base that sits quite well on top of a bucket. Very sleek setup.