I think a bigger room would better serve your interests. For one you really want access to the back of the benches/shelves even if that is only from the left and right sides. Soldering galvanized will be problematic, you might be better off buying large sheets rolled up to minimize seams, then rivet those seams.
As for room size, making such a room is expensive and frankly storage inside the room might not be a good idea. I'd only place critical storage inside such a room, maybe a tool box. These days room for a large screen TV doesn't hurt either.
It might be worth considering an L shape arraignment of the benches as they make movement from one bench to the other easily. The worse aspect of an L shaped arrangement is the access to one corner. Also you can extend storage shelving off one bench end and free up wall space on the opposite side.