Your best bet would be a wire feed that uses gas. It's much cleaner and easier to weld sheet metal. If that's not an option for you, the flux core no gas type will work, but it's messier and a bit harder to control.

 

Basically like CR-Snow said. Flux-core wire actually works a little better for welding on rusty metal like you might run into welding onto old metal AND it works better (than gas) for welding outdoors in breezy situations. But overall, I would personally look or a welder that has gas *Capability* even if you don't use it now. Once you get a welder, if you're like most guys, you'll discover more projects and uses than you expected. I use mine more than I expected. Gas welds are much cleaner, lay flatter, save time not having to clean up. I've used both, but I personally never use flux core wire on my welder ....but I have on other people's. It works but not as nice of welds and a lot more sparks. But if a guy had to weld pipe outside on a windy day, the flux core would be the way to go. As it is, I weld outdoors a lot but if it's windy, I just prop a piece of cardboard to block the wind. What I'm saying is, using gas, I've always been able to make things work out. Lower voltage welders work nice on sheet steel, up to 3/16" ...maybe a little thicker too.

You might also want to invest in a welding blanket from Harbor Freight. They work great for welding inside the vehicle. Save you a lot of work. Be careful not to let welding sparks or grinding sparks hit the glass b/c they will instantly imbed into it and screw up your glass sure as can be. So, use the welding blanket, or some plastic over the glass. A lot of guys overlook that and are sad to find out afterwards they ruined their side windows and windshield. Avoid more work, right??

 

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This is what I have for a welder. I am doing floor pans in my '97 TJ with it, and it works. Cutting the rods in half helps. I am just doing a "spot" weld every so often, then will seal it all. I know a wire feed is far superior, but this is what I have, and it does the job. Please delete my last post. Thanks.