Workshop expansion?

A major project for next year is replacing the roof of my shed. The plywood deck is, um, bad.  Being a 10x10 shed, it can get a little cramped in there.  I'm considering adding a 6 inch thick concrete 4x10 slab tied into the existing slab and then building a 4x10 extension on.  Benefits include more workspace, adding a window large enough for a small AC unit, and since I have to replace the roof anyway (and I still have insulation left), minimal additional cost.  At the same time, I'll run an additional 15 amp circuit to the shed.  Debating whether I should pull off the existing wood siding (which is rotting) and replacing with new exterior grade stuff.