Hope my answers help you out...
Buy the best machine you can afford. There is a huge difference between a quality machine and a budget one, especially on thin gauge metal.
Not necessary to buy an expensive welder. You can buy quality name brands in the $800 - 1100 mark. Lincoln, Miller, Kempii, UniMig all make nice welders in this price range. They are all quality brands with quality wire feeds. Lincoln have a good unit in that price range. Go for a transformer core over a an el cheapo inverter MIG, will work every day all day without burning out for the next 30 years
Most of the brands mentioned are high quality USA built machines. Stay away from gassless MIGs, you want an argon mix for panel auto work.
Also, get on Eastwood and get some of the copper backing tools, they'll help control the heat in the panel and they'll help to keep you from burning through, the weld also doesn't stick to the copper.
yes it works but you can get much better consistent welds without using copper backing plates, they are for people who cant weld sorry... or for people doing bodgy stuff like plugging huge holes with welding wire (who also cant weld). If you can weld you dont need plates. Practice without them and that will teach you how to set up voltage control and wire speed to suit what you are trying to do. Practice makes perfect and cleaniness and the gap between the 2 parts to be welded goes along way to producing nice welds. Like everything prep goes a very long way. Copper plates have their place, but I wouldnt recommend them as a step to learn how to weld.
My first suggestion is don't weld patches in your body with MIG as it's brittle. TIG is the best way to go.
Not entirely true, I use both Oxy gas (softer than TIG) and MIG. Both welds will be hard unless you planish the weld (work the weld a hammer and dolly) You wouldnt use a TIG or Oxy for panel work unless you have very good access to the rear of the panel to planish the weld and stop the weld line from distorting the panel. Generally
you'd want the panel off the car to use these methods. MIG is a very good compromise for welding when is isnt practical to get to the reverse side of the panel. Yes the weld is naturally harder but not brittle. A MIG weld can be planished just like an Oxy or TIG weld but takes longer. MIG is the way for lots of reasons... plug welding (spot weld replacement), low distortion, better suited to structural work and generally speeder process as a lot less after work is required... it is a very diverse welding method.
As for staying off thin stuff, I say rubbish. Thin scrap sheets will teach you technique, settings and control. The best thing you can do is enrol in welding classes, that will give you a better idea on what needs to be practised, differences in various welders (so you know what you really want to buy), techniques, and you have an instructor to guide you correctly.
I have used various methods in my build thread. Most recently I was using an Oxy on the rear panel because I removed it from the car. The panel was then plug welded (MIG) back on the car. If I can do it anyone can do it , welding is not that hard with guidance and a little practice