Are you able to get one built? I've just draw up something similar for a mate. 1700 drawbar, 1500L x 1200W x 1000H box, integrated kitchen, rooftop tent, matching track and wheels etc. We're having the basic hot work done by a welder. Then going to do the cladding, wiring, kitchen, licencing etc ourselves. I've done this a couple of times and find a good way to get what want for reasonable price. Have look here
http://www.expeditionportal.com/forum/index.php for some ideas. The Yanks love these sort of trailer, known as an expedition trailer.
Edit. We're looking at about $6000 licenced, lots less than "professionally" built unit. Ours may not be as polished, but will be more than functional.