If they're just round holes in mild steel, then the Hougan Rotabroach hole cutters with a regular 3/8" drill should work for you. They are available individually in sizes up to 3" and make some of the cleanest holes I’ve ever seen in sheet steel. You really don't need to apply very much force at all for them to cut.
I bought a fabricators set from Colonial Hardware a few years ago, so I know that they sell them.
Black Book of Tools 
