I've watched a youtube video where a guy shows how to weld aluminum with MIG using the standard gun, not a spool gun. IIRC he recommended a particular wire that was stiffer than the normal aluminum so it would push thru the tube and gun without buckling. Might be worth a try if you can't swing buying a spool gun.
Unfortunately don't recall the link to the video - Just ran across it in youtube browsing one day.
