I'd go 220v (or at least something 220v capable), gas, either Lincoln or Miller. Gas shields the arc from outside contaminants and gases which may react with the weld. Flux-core is evil, at least with sheet metal. The big concern with welding sheet metal is warpage. With gas you can (in theory) weld faster and do less warpage aswell as minimize corrosion. I've seen both flux-core and gas, just fork out the extra and money and go with gas it's worth it in the long run.
PS: Check out
this welder from Lincoln. They're about $600 and it comes with everything you need for a gas setup minus a tank. I wouldn't f*ck around with anything less myself.