I use a Hobart Handler 120 for most of my panel welding. For heavier welding I use a Hobart TigWave 250 (it can do stick, MIG, and TIG as well). My Handler is setup for .024 solid wire with straight CO2 shielding gas. This gives me the best weld on thin guage sheet steel without burning through the panel.
When welding sheet metal, I keep a bucket of cold water and a rag nearby to keep the panel cool as I go. Minimizes warping that way.
Brian