Scott Smith of Southern Q Smokers & Trailers (southernqsmokers.com) specializes in equipment for competition BBQ. You can pretty much spend as much as you like on a porch trailer, as each is custom built to your specs. For a comp trailer, you can do as you like, but if you intend to do catering or vending your health department will control what you put into the trailer.
If you're going that route, best to check with the HD first, because it's a lot easier to build it right from scratch than it is to retrofit an existing trailer to meet the many (and sometimes ridiculous) requirements of the government regulators.