I am sticking with plastic, I have seen what the stainless look like after they come in contact with a trailer on uneven ground, the plastic are sooooo much cheaper, they won't rust and they have some give to them. I think the silver color will look decent, I will post some pictures once those samples arrive. I agree, the 1/2 fenders are the way I will be going, if there was more truck behind the wheel, I might consider going with the full fenders.
I will have to measure and take a quick look at the layout, your idea about having the floor area free at the doors would be a good one, but it will depend on where the uprights land, definitely worth investigating.
The 2x4s will need to be over 8' to fit, but I am thinking I can add a vertical 2x4 in the middle, that way the cross pieces could be shorter and I can make some plywood gussets to strengthen everything up. Hopefully there are 2x4s in the quonset hut like I think there are. I still have to buy two more shelving units, basically it costs me $100 a shelf, plus plywood, but I have plenty of that for this purpose. I saved all the cutoffs from when I made the last round of shelves, they will end up being two pieces (leftover is 2' by 4' ish), but it won't be a problem.