Hey vwjoe,
I do a lot of classic car restorations and the sheetmetal will give you fits unless you prepare. You don't need a lot of power...I use a little Miller Sidekick 90A MIG for all my sheetmetal work. If you are going to also weld into heavier material on a vehicle, going with a 140-180A welder will suffice to do everything. I use .023 ER70S-6 wire, C25(75argon/25CO2) at 15-18cfh inside and 22-25cfh outside. I use 1/8" thick aluminum backing strips behind the panel seams to avoid burnthru and tack every 3". The other technique I employ is to tilt the MIG torch at a 45deg. angle to flatten the puddle as I connect the tacks. I use the "push" method when welding panels. It takes some serious practice on scrap pieces to get the "feel" of the beads on panels. You must exercise patience and do a little at a time to achieve excellent results. Once you get the hang of the doing it, the job will go nicely and you will never have a problem with thin sheetmetal. I know a lot of gents who won't do sheetmetal or auto panels because of the thinness of the metal. MIG will give you your best results IMO. I'm sure others will chime in and offer their thoughts. The gents here are the best providing information, guidance, and "food for thought". Good luck....Denny