Sheet metal welding you say? Lincoln or Miller make good entry level machines.
I have a Lincoln SP-100 that I bought new about 25 years ago. It works as good today as the day I bought it and you can buy one now for about half of what they cost back then.
Be sure to get a GMAW (Mig) machine that is equipped for use with backing gas. I use .023 wire with 75/25 gas.
http://www.northerntool.com/shop/to...WXczx8GeQZwZdXewenj57Zubj7YP_tEXs8_w_wcBA SMAW (stick) machine will be of no use to you whatsoever for welding sheet metal.
Good luck and have fun.
John