The flow rates are going to depend on what kind of tank it will be. FO
will not require as much as FOWLR
or even more still if a coral tank. You also need to decide if you want to eliminate any visible powerheads in the tank and rely primarily on the returns from the sump or closed loop.
A 300 GPH
overflow rate would be kinda low for that sized tank. You would still need a few powerheads in the tank though. If that works for you then it's all good. A MAG 350 would do for a return pump. It has a flow rate of about 250 GPH
after head calculation.
I would actually suggest getting something closer to a 600 GPH
overflow though, higher if you'll be keeping SPS
's. You can then get a MAG 7 or similar return pump and have much less need for intank powerheads.
It really comes down to what you want in the tank...