When I was shopping for a MIG welder last November all the ones I looked at, Lincoln, Miller, and Hobart, recommended a spool gun for aluminum. The problem with aluminum wire is that it isn't as stiff and can "bunch up" trying to go through the 8 or 10' gun cable if it's mounted in the welder where you normally mount the wire spool. I had no interest in welding aluminum so I never inquired anymore into the spool guns so I don't know how much they cost.
It can be done though with a standard MIG welder if you set the machine up correctly. Here's a
link about welding aluminum that was given to me when I was asking about MIG welders on here. 100% argon is the correct gas to use.