I recommend the Lincoln 135 amp MIG unit. I bought mine off ebay for less that $400 and see them at Home Depot/Lowes for about $450. Been in use for almost two years with zero problems.

The wire feed MIG units operate best with the inert gas so get a unit that has the gas regulator included. Flux core/innershield wire is okay but for sheet metal work you will want to use the MIG process which requires the gas.

You can buy the gas at your local welding supply but you will probably have to buy a tank to start with and pay for refills after that. The buy cost for the tank should be in the $130-$150 range and refills are about $20. A tank lasts a good long time though.