I can echo Scott's comment about the noise generated by the Marco pumps. I use one for transferring diesel fuel in the boat and while it is very good, it is very loud, likely because of the gear-driven pump setup.
I did not know the March pump with a brushless motor was 4mm. Mating that to the 8mm Flojet diaphragm might be a challenge, but I am sure there are adapters out there. Since that motor is already functional without adapters and programming, etc. I think it would be worth a shot if I can locate one for sale. So far, I have not seen any for sale, except maybe through the manufacturer.
There are actually several March pumps that use a brushless DC motor (12 and 24V). Given its higher capacity (and presumably more powerful motor), I was looking at:
Phantom, A54 #44