RE: building a hot dog cart
I've pictured that project myself. I think the only plans are the ones you would layout on paper. As for the construction angle, make sure you have everything squared and secured. Be a shame going down the highway and watching the 55mph winds tear it apart! One thing that I have seriously considered, if I ever was to do it, is use those new metal studs. They are extremely light, once screwed together they are stronger then 2x4's. Price wise, they are comparable to lumber. Most expensive part would be having the stainless steel fabricated (cut, bent, etc) for the interior. I'd probably start off by contacting the health department for all your electrical, plumbing, flooring, walls, you name it requirements. I dont know if it pertains to a mobile food cart, but a Chef freind of mine told me any new food establishment needs to submit a site plan, right down to the shelving before one can build. I haven't investigated this, but something worth checking into.
A good base to start would be a low profile, 2 or 4 wheel trailer, or trailer frame. These are actually pretty cheap new, and even cheaper used. (Get an old popup camper and strip everything off the frame).
Hope some of my rambling here helped you out.