Pretty much any pump will be fine, you'd be hard pressed to find a pump that struggled with any amount of water in a loop, and i've seen some pretty crazy amounts of water. Flow rate doesn't have much of an effect on cooling, since the balance is between the water having enough time to get heat and the water not getting to hot. The thing to bear in mind though is to get a good quality pump, not just one because it's cheap that will break in a year.