If the MM 250 you have was an X model, you can attach the spool gun directly to it. If not, there is a kit you can purchase from Miller PN 042607. This will easily install in your machine & you can plug the gun directly in after that. The Spoolmatic 30A is going to run you $450-700 used depending on where you get it from. Keep a lookout on ebay & you can probably get one for a reasonable price, $500-600 is around average. If you try using the regular gun you have to run AL wire, you will have lots of problems & get frustrated fast.

Good luck on it. Steve.