Probably the easiest to use would be a small MIG (wirefeed) welder. You are limited with the 110 power requirement. For thin sheet metal its hard to beat a MIG. Harbor Freight has several Mig welders reasonably priced although I would stick with a Miller or Lincoln. The power supply and wirefeed are superior on the name brands verses the import welders which make it easier to produce a quality weld. I currently own a wirefeed, gas and tig/stick welder and find the wirefeed the most usefull unless I am welding aluminum which is where the tig comes in.
Most of the good wirefeeds will weld either with a gas shield or gasless using flux core wire. Using .030 wire you have a pretty good range of thicknesses you can weld with. Good luck.
Mark