I'd second Steve's comment about going with a name brand.
I have a Lincoln Weld Pak 100 that I've used for 12 years or so. It does fine for my needs, however thin stuff like sheet metal can be tricky. Mine has infinite variable speed, but not infinite voltage (heat) settings. I only have 4 heat settings and sometimes feel like I really need one in between. They cost more, but if you can find one with infinite voltage I think you'll be happier welding thinner stuff.
One more thing, get one with the gas conversion option. Many in the $400+ range are flux core and then have the capability of adding gas. I used mine for 6+ years w/ just the flux core, but once I finally added the gas conversion I never went back. The welds are much cleaner and easier to work with.