I agree with the above statements... Voltage and wirefeed adjustments are pretty much a must. Also, try to get a 220V welder, rather than a 110V. Can use it for welding thicker metals, along with the thin stuff.

I'd also recommending picking up some Kleko's (Temporary rivets) if you plan on welding in filler panels. Helps clean things up, rather than using regular rivets, and then drilling and grinding them out, just to get a smooth surface.

I hate to be Debbie Downer, but if you plan on doing this cheap, I think you're over your head... You can try to see if someone local has a welder and will teach you/let you borrow theirs. Don't start on your car first though, if you haven't welded before.