20x your tank volume... 75x20=1500gph to start with. I'de stick a maxijet 1200 (295gph) with a plastic pre-filter in the bottom right hand corner of the tank behind some rock to blow through the back of the rock and through it. That will help detritus from building up behind the aquascape. Then I recommend getting a couple of Koralia Evolution 750s, one on each side of the tank.One on the left should point across the top of the water, and the one on the right should point towards the front glass and around the corals. This will cover most of your dead spots, depending on your aquascape.
Internal flow rate really doesn't have much do do with your return pump. Return pump and water flowing through your sump should be sending clean, filtered, and heated water back to your display tank, however the internal flow rate prevents detritus build up and dead spots in the display tank.