Mig Welder Question???

[StormRider]

I am thinking about getting a mig welder so I can do my door skins and my rocker panels on my truck. Plus I think that it's just real handy to have one around. I don't too much about mig welders though. My question is with so many brands and so many different amps and volts and gas or no gas that I am confused on what to get. I guess my budget is 400 or less if that is possible. Would a 200 one without the shielding gas be able to do the welds that I need. What is that shielding gas used for anyways? Thicker metal? More heat? Any one know of a good place to buy one online? Anyways, was just wondering if any of you experienced welders out there could point me in the right direction. Thanks.