In my opinion they are really, really well built and would make a great camper base. They will put up with any abuse you can throw at them.
As you already asserted they are very, very heavy for such a small trailer, they may even be made from in part from lead lol. Because the trailer itself is heavy it really limits what you can put in / on it and stay within GVM, especially loaded up with water, food, beer, beer and beer. At least for a poor lil TJ.
Wish you the best of luck on the build, i had the same plans in mind and did buy one from auction. But, i ended up getting a bargain buy on a real camper with all the fruit so the poor #5 is just sitting in the backyard getting rusty these days