I have welded aluminum with MIG before. You don't have to have the spool gun, but it probably is better. I got a teflon liner and ran it through the machine. It worked pretty good, TIG is better, but you can get by with MIG. Aluminum needs a good bit of heat to start with, then you drop it down. That is why TIG welding is better if you have a foot control to control the amperage while you are welding. I did it the way I did because I was just checking it out and wanted the minimum investment. Get a teflon liner, aluminum wire, and pure argon gas and give it a try.
Hope this helps,
Alan