Anybody done it ?
Lots have done it, and I think it's a great idea.
My brother has gone this way for his current camper trailer. Full offroad trailer built to his specs by a local mob. OzTrail Camper9 tent on top of it. Basic, but everything is solid. From there, he has added on gas rings, toolbox, jerrycan holders, pole holders, built a kitchen, etc - everything else he needed. Allowed him to get a rock-solid base for not much cash, then add the extras on himself as funds and time allowed.
It allows you to get the exact setup you want. The only thing I'd advise is to think hard about what inclusions you want for the basic trailer. Personally, I'd get it with brakes from the get-go, but Grant had his made un-braked, and bought a kit from ebay and welded them on when the weight of the trailer started to get up. So either way works. Also, consider the tent you're going to use based on your space needs but also on how easy they are to set up. I myself find the annex setup on the OzTrail tops fiddly and difficult, but they're a lot better than some other ones I've seen, so...
Good luck - make sure you start a My Camper Build thread if you go down this path. I'm always amazed at the good ideas that people have when building their own campers.
Cheers!
Matto