If anyone can point me in the right direction to work out how many layers I will need to use of what weight that would be fantastic.  I do realise that manufacturing technique plays a significant role in the end strength and so there is not a single right answer but I'd like to have a directional understanding to get started.  

Many thanks
Nathan

Mate, I appreciate the help but chill a while.  I didn't ask for an answer to how I should do my layup, but a pointer towards resource to help me learn about the current state of materials stuff do the calculations myself.  

The load stuff will be fun to work out - the box will be around 2,500 to 3,000 kg total seeking to be broadly evenly distributed otherwise over the floorplan.  But when you factor things like 300l water tank, large batteries etc, there will be a few fairly uneven load points that need consideration.  It also needs to be designed so that it can be effectively supported safely at the corners by jacks, on the subframe on the vehicle, on a fork lift and even possibly from top load points with a crane like a container lift (although this probably adds more complexity than practical value!). 

As I get a bit more into the details of the design it may even be the sort of thing that doing a bit of FEA load distribution work could be useful if I can get my head around getting the most out of my CAD package.  

A few decades ago in the pre composites era I was an aero engineer and did my fair share of structural design work so, whilst I'm sure I'll be super rusty on the maths, I do have a bit of an idea about the load side of things, I'd just like a pointer towards a recommended accessible resource on the materials side to help me learn about that so I can find my own answers and start a practical testing regime knowing I'm close to the final answer if not exactly on it.  

In terms of foam, I'm not yet certain what I'll use - again this is new territory for me so I'm acquiring knowledge.  I had looked at Airex C70 so it's helpful you reference that as a material of note, thank you.  I will probably use mixed density in different parts of the build depending on the mechanical requirements, as you point out each panel has very different needs.  

Part of the fun of this project for me is that I think that most off the shelf solutions are far from optimised and just use a one product fits all principle - for obvious reasons for mass market builders.  Whilst I'm not looking to create a race car or satellite, shaving every excess gram, I love the idea of being able to think more critically about design and do a bit more optimisation.  

I thought that a community of folks interested in composites might have a recommendation of some learning resource that's all.  I often find internet communities helpful and welcoming for interested newcomers so thought I'd try it out.