With the proper size Mig and gas shield, you can weld almost anything. We just rebuilt the back of a specialized truck with a Mig. That was after we cut it apart with a plasma cutter. A lot of 1/2", 1/4" and 1/8" plate. I'm using a Mig to do my frame work on my 'burb. Our shop is full of old stick welders the size of a Yugo and about all they do these days is gather dust!
http://www.bunchobikes.com/my1929chevy7.jpg The machine I use is in the background on the left. The one on the right has an aluminum spool and is set up for welding aluminum.
If you use a small machine, don't expect to do heavy work! Don't expect to find a decent machine at the big box stores or HF. Go to a dealer that sells Miller or Lincoln directly. With that big ol' Lincoln that I use, I can do heavy stuff or set it for sheat metal.
