Thanks for the suggestions. I actually spoke to N&J a few days ago, and they came up with a quote for me for an aluminium box, but it was very pricy (around £5k with the VAT), without any shelving or other bits and bobs. I may try and simplify the design and get them to re-quote though. I guess the fewer corners and angles the cheaper it will be. I've also spoken to a company called Anglesey Camper Conversions. Does anyone know anything about them? I'm going to ask them to quote for something like the last pic that rosco posted. They specialise in pop tops, although I'm not sure how big they are on metal fabrication. A pop top would be awesome tho.
Some of the slide on campers in oz are made from aluminium sandwich foam built over a steel frame. I havent worked with sandwich foam before,but it maybe easy and cheaper.
Some more ideas here
http://compare.ebay.com.au/like/110953805628?clk_rvr_id=389634189131