The size pump (flow rate) really depends on what fish you want to stock.
If you want swamp fish, or placid water fish like discus, or Bettas you can get away with lower flows, less GPH.
If you want fish that come from streams, or riffles that require a higher oxygen content, and prefer moving water, such as Geophagines, Tomoichla, African Tiger fish, or other riverine species, you want a pump with plenty of power, and lots of flow, plenty of GPH.
Or in a similar case you can supplement the sumps pump with power heads.
On my riverine cichlid system of a 180 gal with sump, I use a 1500 GPH pump in the sump, but add a power head in the main tank to create strong riverine movement along the length of the tank.