For 500 USD I still can't help to think that patience could score a solid, full sized AC/DC tig welder that could also do stick. As tig has been around even longer than mig (50 years or more?), all the companies have made their version. These machine are capable of welding anything that can be welded, and don't have all the moving parts of mig.
That said, how about one of the Hobart handlers:
http://www.northerntool.com/webapp/w...73&R=200306073
http://www.northerntool.com/webapp/w...75&R=200306075
Note the difference in duty cycle. I think that is one of the main things the extra money buys you.