As another newbie I'm probably (certainly), not the best person to answer, but i have made quite a lot of other stuff, most of it lightweight.
There is very little room for supporting structure over the wheels on a 32 and I'm guessing you still need some sort of framework to attach the sheeting to.
If I were doing it I would look at a couple of options,
1. go for a frame of 6mm steel rod, bent to shape and braced as much as you can, and which is a skeleton for the shape you need. I can't weld, so I would opt for no 2.
2. go for a laminated ply strip structure, again just a skeleton, but with the main side profile rail from nose to tail supported at regular intervals with vertical strips.
In both cases I would start with a base plate that sits on the chassis, though there will be complications at the places where the axles steering rods etc. cross the rails. (Unless your axles go through or under the rails).
Once you have something that sits on the chassis you can build up with risers and braces to the shape of the top rail that supports the top bodywork. It's also likely to need some cross members to hold it rigid, and again diagonal bracing. Think Birdcage Maserati.
I am currently working on using bulkheads at the front, steering column, and behind the driver to provide necessary structure, in your case if they were mounted on the base that sits on the chassis it would give key support points for the sides, though a separate independent steering column support would be needed.
Making it an opening clamshell might just give you more work than a lift off body, as you then get into hinges and extra structure to stop it flapping about when not attached to the chassis.
I think I would split it into a front section up to the front of the door position, and a back, to be less unwieldy? Not sure how often you would need to get at the front, but I'm guessing it's more likely that access to the engine is needed, and you wouldn't want to have to take the whole body off every time.
As one who knows naff all about these vehicles, but who likes lightweight structures, have a think also about 20mm plastic overflow pipe for support, there are several fittings available, though whether or not it would be rigid enough is up to the pro's to say, but I'm guessing that with the three bulkheads theres not going to be too much flopping around.
As I said at the beginning, I'm not the best person to ask, but it's good to know there's another builder on my doorstep! I'm still at the stage of printing out 200+ sheets of A4 to get a full sized plan, elevation, and sections for mine.
Peter
PS might you need an under tray to stop a lightweight body like this lifting, there's not much inherent strength in Corex if it gets airflow pushing it up?