Well as you posted this in the tig welding section I'd hazard a guess that your after a tig?
Stainless and mild steel up to 3mm thickness will all be dealt with below 100 amps so a 240volt domestic supply would be what you want to hook it too?
Going by the two materials you have asked about I'm also guessing a DC only machine is what your looking for?
As for budget my crystal ball can't extend that far but you can get a nice unit for 400-1000 pounds.