I would look for an all aluminum cargo trailer and work off that platform, if I wanted to totally custom build a TT. That would get you a new shell. I investigated this exercise before I custom built my old Starcraft. Even though I can (and have) build everything about a trailer, buying a built frame, running gear, and body, is cheaper than doing it all myself. The least expensive manner is buying a used trailer with good running gear that is in bad shape and gutting the inside.
If you start looking through the build threads of builders starting with a frame-up build, I think you'll find that even steel construction trailers come in over budget when all is done. Your weight allowances push towards an aluminum frame and that gets into specialty welding.
Have you looked at the Camplite trailers? I think they might sell a model that is close to what you're after. If you put a lot of factors on the agenda like work time, resale value, immediate use, etc., it may be a better solution.
If you do go ahead and build DIY, be sure to post back with your progress, it is interesting as heck to see what others do. Good luck going forward.