Old Toyota Odyssey 1988 restoration

Hi All, 

I am the very excited owner of a properly dilipated Toyota Odyssey import from US. Engine and chassis are not in a bad state, but body is badly affected by damp. 

It needs a lot of work but priority 1 is to stop the water ingress. 

Roof is made up of gpr, with some styrofoam for insulation and wooden struts plus steal frame. The wood is badly wrotten and the metal rusted. Side walls are mostly ok apart from the wall under one of the windows which is rotten through. 

I am deliberating what to do with the roof. 

There are two distinct routes I can take. 

1. Try to make the new roof slightly convex so it does not collect water. To do this:

Make roof rafters by taking fairly large cross cut battons say 12cm, and tapering them to 2.5 cm at the edges to achive slope. Use 5.5mm plywood to cover them on top. Insulation in the middle. 3mm ply at the bottom

2. Go with the flat roof, clean the tar (doh) prevous owner has used from the fiberglass outer, put new rafters from a) wood or b) aluminium and insulation in the middle and 3mm ply at the bottom. 

Option 2 should keep overall weight down, but will create risk of pooling water again. 

In both isnatnces the final roof finish will be liquid e EPDM rubber. 

I will not use the roof for storage or walking

Another option I am considering instead of using plywood is to use GRP sheeting. 2mm x2400mm x 1200mm costs around the gbp 90 mark vs around 12gbp for 3mm ply. But even if I get a leak in future the structural rigidity of the roof will not be affected and
in theory it will be much easier to fix.

In terms of aluminium profiles vs wood rafters, aliminium is lighter, will not rot. It is not much more expensive but it is hard to work with. 

Finally what insulation thickness to go for in the roof? I am contemplating between 25mm celoflex rigid boards and 50mm. 

Hugely excited from what is ahead. 

Looking forward to hear what you think. 

Nikolay