Have been thinking about this too...
New to the forum, but have been thinking about this a lot -- have a family of four and we like to camp beyond the pavement and are busting out of our 91 Range Rover.
But I think I'd keep the frame and axles, etc... and remove the bed, which is ridiculously over-built for the amount of stuff any of us would ask the trailer to carry. From what I can tell, the bed alone is 600 or 700 lbs, and unbolts from the chassis, so was thinking about getting rid of the bed, and using just the frame, so you have a stout chassis frame with brakes and pintle hitch to start with. (The m716, the flat-bed version, is 740 lbs in A2 form, and still has that heavy flat bed mounted on the frame, so I think the frame alone must be 100-150lbs less than the 716.) That would make just the frame with axles, brakes and wheels around 650 lbs.- heavy, but not totally over the top. It would also be a little narrower, as the bed is wider than the wheels rin-to rim. Supposedly the wheels are just under 6' outside-to-outside, about the same width as my classic Rover, though the track is probably actually a little wider.
Then anyone with some woodworking or welding skills can build whatever you want on the frame -- from an offroad teardrop to a Campa or Conqueror-style trailer, all for about the same weight as the original cargo box. The total trailer could be 1300-1500 lbs in the end, which is too much for technical trails, maybe, but not bad for rough-roading.
As money came in, I thought about swapping axle and springs, but to start, might just remove a couple of leaves to soften the springs up a bit. I like the strength and reliability of the mil-spec axle and leaf shackles.
But to start with that frame, axle and chassis for $400, it's tough to beat. To have that built custom would cost 5x that. Even if you could weld your own frame, the axle, springs, wheels, tires, hitch, brakes, etc... would cost a lot more.
Thoughts?