In saltwater, bigger is almost always better. The minium size of your sump should depend on several factors:
Do you want a fuge in the sump?
Do you want a skimmer chamber?
Do you want bubble baffles?
How much backflow do you expect during a power outage?
How much evaporation tolerance do you want?
After you figure out the minium space, consider the greater stability excess water volume lends to a saltwater system.
As for the pump, measure head from the water level in the pump chamber to the water level in the aquarium. Find the power curve graphs for the pumps you are considering, and find one that pumps 400-600 GPH
(6-10 GPM) at your head rating. I personally think you're better off with an underpowerd pump rather than overpowered. You'll have to suppliment your flow rate anyway, and an overpowered pump will be noiser and burn more electricity without much benifit.
If you want to keep softies, your total goal flow should probably be around 10x, or 750 GPH
. Since your overflow can only take 600 GPH
, consider a closed loop or powerheads to cover the diffrence between your sump flow and your goal flow. Since closed loops and powerhead don't have to worry about head, just add together thier ratings with the sump flow to see how you're doing.