yes you can get a nice MIG for $400-$500 but most will have little or no aluminum ability. I personally own a Miller 135 which i bought for $445 and have been extremely happy with it. As for applications in FIRST your team needs to decide if you are willing to limit your building materials to match you welder.
There were two excellent threads a while ago talking about welding.
http://www.chiefdelphi.com/forums/sh...ad.php?t=38730
and
http://www.chiefdelphi.com/forums/sh...ad.php?t=30679