As far as the stud question, what daddy said.
As far as the cargo door question:
I have built about 2 dozen food carts/trucks in the past few years, about a dozen of those cargo trailer conversions.
If your serving out of the rear the easiest and most efficient, and pretty much the best looking, method I have found is to frame in a partition wall with a window in it. Exterior side of the wall can pretty much be paneled over in anything. I sent one to Canada that was stainless exterior, build one for Portland OR that had cedar lap siding, you get the point. Build a 2X3 wall, frame in the window, insulate, run electric if needed, panel over. Easy as that.
On top of that method being easy, at the end of the day you can close the doors for security. Oh, and the doors when open are great for hanging menus on.
If you are not serving out the rear, stick a couple screws in to make sure they never open and pretend like they aren't there.
<message edited by a.mckenna on Wed, 02/13/13 10:33 PM>