I was always curious how these pool filters would hold up to marine conditions? Anyone using them do anything special to ensure they don't rust out from saltwater use?
You could buy or make a fluidized bed filter. They handle a giant bioload for their size, inexpensive - but steal O2 and can return anoxic water in to system if not areated. THey can also crash with power outages, and changes in pump flow.
I am a big fan of bead filters for their stability and easy backwashing. They allow a much higher flow rate and drops are not a crucial. This one could cover your planned system. It is a basic but popular model. I have had two of this brand with no problems.