I'd recommend something like Styrofoam. I bought a box of twelve 25mm thick sheets. Each sheet was 600mmx600mm. It's expensive however it ticks all the boxes. I honestly cannot remember when I bought mine (it was a few years ago) but a quick google search
reveals this website This is the material I'm using in
thread over here. I'm not a good modeller at all but I've found the stuff forgiving enough for me. I've glued it down to 18mm MDF, and last night I glued 6mm MDF strip around the sides to protect it. The surface itself is pretty easy to mark- this is ideal during the building stage but less so when you are read to play on it. In the past, just before painting I've painted watered down PVA glue across the whole board and once it was dried used some of Captain Blood's patented Basing Gloop (TM). This gloop is a mixture of sand, white glue (PVA) and paint. You could probably forgo the watered down PVA as this stuff dried rock hard.
A few years ago I used insulation foam from a DIY store to make
these boards:
The insulation foam seems to have completely disappeared from sale. I purchased it from B&Q in Britain. There where a few oddities with this board. The size for one- I cannot remember the exact dimensions but two of these sheets, laid together, measured approximately 4' by 3'. I much prefer using the 600mmx600mm stuff mentioned above.
I did learn a lot using the above material. In particular that without further treatment (such as strip material along the edges) it doesn't last particularly well. Now I'm pretty sure that my handing of it has been rougher than some of the other guys on here. Especially since in the last 5 years I've moved house about 5 times! With at least two of them being 150 miles. It's also only recently that I've gained a safe storage place for all my gaming stuff.
During the construction of these boards I've found that a Dremel, a sanding block (I quite like the disposable foam ones- you can manipulate them to different shapes), and an air brush are must haves.
During my current construction I've realised that there are some other tools which would have been very useful; especially when attaching the strip material. Mainly sash clamps and a try/engineering square.
I think your plan looks good. I'd just ask what the scale of it is? How big is the cavern going to be for example. Where are the board divides going to be? I'd recommend accurately drawing the plan out using square paper, cutting out each tile and checking to see how modular a design actually is.
Cheers
Andy