Welcome to the site and good to hear you are excited about a build. Frank has a set of plans for a 24x48 on the home page that you might want to check out. It is a quality pit and a great reference even if you don't use them.
You'll need stainless 309L mig wire and argon gas for welding stainless to mild steel. It can start to get expensive if it's the only time you'll use it.
I live in a wet climate (England) and all of mine is mild steel. Once cooked on a few times and seasoned, it will have an oil/smoke layer on the cooking grates and inside of the stack, and the outside can be painted or BLO coated. Mine's been used in rain at least a dozen times over the last 12 months and no sign of rust anywhere.