Go with filter socks rather than foam. Best mechanical filtration I've ever used. You can find them in varying micron sizes, in fact they can filter everything from 200 microns all the way down to 1 micron if you choose. That's better than a DE filter. Just buy 3 or more.
Why you need all the foam in-between the chambers? Just have the drain pipe going into the filter socks.
Look into getting a media reactor, check out's Bulk Reef Supply's dual chamber reactor. I use it as both a bio and chemical reactor. My first chamber is filled with Seachem Matrix and the second with Purigen or carbon if I need it.
Also, implement a way to drain the sump to allow you to conduct water changes at the turn of a valve or switch of a second pump.