Parts are unobtanium new. You can get a second 'Nest if you need major parts (sliding side windows mainly, which I've not had a lot of luck finding new). Or rebuild it yourself, they just use common hardware store stuff and straight up fiberglass techniques. It ain't rocket surgery. The only part that takes some skill and equipment is the tent. If you don't have an industrial sewing machine you probably will have to find someone to make it for you.
By weight limit I assume you mean the maximum weight on the bed. No, not practically. WilderNest recommended a max of 1,000 lbs on the support cables so two adults shouldn't seriously stress the camper.
However, the one key to this is that with the top flipped open all the weight creates a moment that puts a lot of the force in tension on the driver's side bed rail. So take care to bolt it securely and use washers. In normal use the weight of the truck prevents anything from getting tippy, but on a trailer it's possible that you tip it over.
WilderNests have two outrigger poles that were designed to prevent swaying. People often think they are support poles, but that is not the case. The weight is supported by the cables, like a suspension bridge. I don't use the poles a lot of the time. If you have rock sliders it's better to use a Hi-Lift under the passenger side to prevent the truck from rocking. There is absolutely zero movement.
If I was considering putting one on a trailer that doesn't have 2 tons of truck to keep it grounded, I'd probably make an outrigger that stabilized the trailer, like they use on fire trucks and cranes.
Conceptually, something like this:
