Air Punch Flange Tool

If you have any amount of flanging to do, get one. They're worth it compared to doing it by hand. Also, they're typically available in two punch sizes. Get the larger of the two if you can. The smaller one is a little too small to do decent plug welds. You shouldn't have problems with rust if you can properly seal up the back side of the seam. Primer and a generous amount of seam sealer should do it.