Gertie, I'm guessing most people buy the box van, or cargo trailer, or whatever and fix it up themselves, so probably not a lot "done" for sale, and if you do find one fully tricked out, it will be very expensive.
I've been looking at buying a new cargo trailer and fixing it up, and watching a lot of youtube videos to try to educate myself.
I'm no expert by any means, but I suggest doing a search in youtube or google for what you are thinking of, and gradually defining what works for you.
Here's an example of my first paragraph. New 6x12 cargo trailer, about 3k. New 6x12 cargo trailer with insulation, windows, etc, from weeroll (as featured from one of Bob's vids) 7k. Fully redone and tricked out cargo trailer 6x12 15k or more. This is why I will be buying a new bare bones cargo trailer and doing it myself. It seemed intimidating at first, but after watching a ton of videos, I figure once I have done the insulation, and maybe a window, it will be "livable" and I can work with what I have to make it work.
I don't plan on worrying about wiring or plumbing. If I want to upscale later I can. I can throw a mattress on the floor, and I have a great bucket with a toilet seat, so really, isn't everything after that gravy?