I have done body work with flux core and with patience it turned out okay. That being said I would recommend not even attempting that. Buy the gas an some 024 wire and practice. quenching is also handy when doing sheet metal. It speeds up the job, but you still need to be careful. If you can find an inexpensive automatic helmet it make learning easier. I use mine for most projects. unless I'm doing a lot of heavy work then I switch back to a flip lens helmet.

What is kind of welder are you using(make and model). I personally like Miller/Hobarts, but most of the machines on the market now are pretty good.