I knew I'd forgotten something! Meant to reply to your last post with regard to material choice but that got lost among the Christmas festivities.
Based on the design you've posted above, I would go for either Nylon 6 or UHMWPE (or the slightly cheaper, but essentially the same, RG1000) and for a thickness, I'd say 15mm should do the job. I'd normally suggest HDPE as well but I'm conscious of the fact you want it to support quite a lot of weight, and I feel HDPE wouldn't be the best option in this instance.

You could also go for a thickness up to 20mm if you wish/can afford it. A 20mm thick chassis of Nylon would be incredibly tough; the first chassis I built from it back in 2007 was still going as recently as September, and although a few threads are stripping here and there and there have been several weight-reducing chunks taken out of it, it could still go for a little while yet (although its current owner does have plans to replace some parts of it).
I only started using UHMWPE/RG1000 in my most recent build and compared to Nylon, it's not quite as strong or as heavy and it wears down faster but it is still a good plastic and a better choice for your purpose than HDPE. Again 15-20mm for a chassis would give a solid structure, so it really just comes down to your budget.

So; Nylon 6 > UHMWPE/RG1000 > HDPE
and anywhere in the region of 15mm - 20mm thickness.

Just my opinion though.
And you seem to have done your research so I'm guessing you know this, but just in case you don't, Direct Plastics is probably your best bet when it comes to buying the stuff.