Hi, I'm an apprentice light metal fabricator (sheetmetal worker) You'd probably wanna go Mig for thicker stuff, like welding things to diffs, and Tig for thinner stuff. Tig is a finer, more concentrated welding process, and it's often used to do root runs on important welds, like for pressure tanks, and for thinner metal, as well as alloys such as aluminium. To weld body panels with Mig, you'd want the amps right down, and spot weld (tacks) otherwise you will blow (melt) holes through them (I've spot welded thin metal, .8 from memory, it took forever, and I spent half the time filling up gaps haha).
At the end of the day you're gonna pay a lot for these machines, and then you have to rent gas bottles (can't be purchased) and buy regulators, because gasless mig is crap.
You can always do the tafe course, and when you're a good welder, have a crack, or even take the stuff in and have the tafe teacher weld it for you, for free, tafe teachers get bored welding the same old 10mm plates over and over, they're more than happy to show off.
Hope this helps