With the current welding technology getting as good and cheap as it has in recent years, I would honestly just look into a new Miller or Thermal Arc unit. A quality 220 MIG can be had for 800-1000 bucks. With a used unit, you are getting outdated welding equipment that may break or need replacement parts, once all that has been added up, especially if it is something goofy like the gun, you would have been better off buying new.