In reply to original question and nothing else.
Keep the your stick welder, and buy some TIG gear.
TIG is the best for body work, it adds the least heat so will not distort the panel, it is also the most forgiving for this kind of stuff, you will still need to rent a bottle, but you will be able to do a quality job. MIG is the easiest to use in general, but its not the best for THIS job. Don't be put off by people saying it is hard to use, it is just like stick but you use two hands so the transition is easy. A shat TIG job will be much better then a shat MIG job for this kind of stuff, likewise for a good job, so you can't lose. I would suggest that you spend a bit more and get a triggered arc though, it will cost a bit more but you will not have to strike it, very handy in tight spots and it means you can start exactly where you want to. Use the TIG you already have it and it is the proper thing, I know I might be upsetting alot of people as everyone seems to be loyal to there MIG, but comeon, it should have been TIG from the start.
As far as mild or stainless, try to stick with the parent metal all the time, so mild in your case.