There are heaps of variations out there. Here's a couple of things to look at.
* Suspension? It needs some. Parts should be standard so if you need to fix it, you can.
* Does it have everything you want? It should have a bed, kitchen, storage for gas bottles, maybe storage for jerry cans for fuel. It should have a water tank underneath with a stone guard (thin metal sheet, it's enough).
* Does it have stabilisers? You will hate it if the thing rocks and bounces at the slightest provocation. Corner stabilisers that wind down and support each corner will stop that.
* Does it have storage for food/clothing and is that storage dust/water tight?
Also look at how easy it is to set up. Some of the units are as easy as "pull this lever, then that handle, connect this pole here and bingo!". Others take half an hour and require an intricate set of instructions to get it right.
They're usually built on box trailers, and they're usually light, so they won't need load levelers and you won't need to extend your mirrors.
Just make sure you have the right trailer plug for it and if not, adapters are available. I carry a flat-to-round adapter with me at all times, in case I need to tow a trailer with a round plug on it - I have a flat 12-pin adapter on my car, which matches the 7-pin flat adapter too but has another row of 5 heavier pins that I use for auxiliary power and brake power.
Oh, make sure the trailer has a working handbrake.