How to calculate head loss, old so newer pumps aren't on. Note how adding elbows and height seriously changes things. My pumps are labeled 1500 and 780 gph which is well over the 1800 gph I would like but with head loss I am getting an estimated 1200 gph. If you have a CO2 reactor on the system that is 2 90* elbows plus whatever it takes to get it plumbed in, at least 2 more 90* elbows. Search Reef Central for info on the quietest pumps and remember to consider efficiency as some pumps are monsters for flow and don't cost much but are expensive to run. Pumps run 24 hours a day unlike lights and heaters.
I ended up with a Laguna MaxFlo [a pond pump that comes in a space age looking cage] but Rio Hyperflow was on the short list as well. Have had good experiences with the Rios unlike reef keepers and they come with a wide array of attachments which come in handy.
I am sure you have seen this but here is BeanAnimal's page on his design.
Sumps don't need much servicing. I don't even have batting to replace. I wonder if you put the 20 tall on a plank so it can slide in and out when mostly empty of water it would be easier to deal with? Then you could clean out the pump, get the mulm out, get sponges out to rinse and such.
If you are running a lot of water through the overflow then use a flat overflow with no teeth. My 17" long overflow with equally spaced teeth couldn't handle anything near all the water I wanted to go through the system so I broke most of them out and now the pump is barely choked back.
MFK members use sumps a lot. Might look through the info on that forum.