floor rust wheel well

What kind of sheet metal should I buy to patch large holes/ replace Stairs, wheel well area. Has any one reused the sheet metal from the roof on the floors?

I bought a school bus on eBay (site unseen, a 2003 International 3000) and after getting the floor up, I've found that there is very severe rust on the wheel wells and the floor in between. Probably the bottom 2" of the wheel wells themselves are rusted away, and the floor for a couple of inches...