yupp, don't even think about the arc welder. won't work with sheet-metal (can you say "holes" ?) ...
if you never welded before, go have someone who knows teach you (or take classes) before you start burning your car to the ground.
welding is a artform and even more important then talent is *practice* !!!
also, if you have a rusted through top of your framerail (hellhole), you will need to make sure you take care of the INSIDE of the framerail before you patch up the holes, otherwise, it'll simply rust from the inside out.
you need to know how to use things like MetalReady, weld through primer as well as all sorts of cut-off and grinding tools.
air-tools rule for that kind of work!
Andy