When you do the floors, 18 or 20 gauge steel is the thickest/thinnest you want to use. Anything thicker, you won't be able to shape it, and anything thinner probably won't be strong enough. I used a flux core welder for most of mine (started with an arc welder, but that would be way too difficult to do all the floor welding with). Using gas would be your best bet, because you can see the weld much better than with flux core and it is a lot easier to weld thin metal. Butt welds were very difficult to do even with the flux core welder. The nice thing is that a wire feed is basically point and shoot, where an arc welder takes a more steady, skillful hand.
I would suggest trying to find a sheet metal brake, it would be extremely helpful for shaping.