Personally, I do not think the liverock chamber is necessary or even desirable. Without a functional sand bed I think that part of the sump will just be a detris trap and either eventually cause you problems with phosphate/nitrate or just be something else you have to clean. Plus you have room for more than enough liverock in your tank so I just don't see what having some in the sump is going to buy.

I like Dan's entry filter with refugium design and that is what I'm having him build for my 215. It's a very simple design and very functional. Then you could go with 30"L x 20" or 21" W. That would leave you 18" on the side for the external pumps. I like the Iwaki pumps although they are kind of expensive, I think the 40RLXT would work fine.