Restricting the flow does cause it to heat up more usually it is minimal but if you try restricting something like mag 18 back to 600gph your pump will probably be more effective at heating the water then circulating the water.
With built in overflows the size of the pump is not that critical as long as you don't pump more water than the overflow is capable of returning. Don't have to worry about maintaining a siphon like with HOB overflows.
Size will depend more on what you are planning to do with your sump if you are going to just have a fuge then something as little as 200gph would be sufficient if you are looking for additional water volume something from 600-1200gph is probably enough. IMO There is no need to try to turn your whole tank over 30x an hour through your sump if you want more flow without adding power heads inside your tank a closed loop is more efficient for trying to increase the flow rate of your tank then using your return pump.
Pump recommendations I have both Mags and quiet-one pumps both seem to be good and I have no issues with either other than my quiet-one would occasionally stall after a power outage, contacted customer service and they sent me a new pump no questions asked, then soaked the old one in vinegar for a couple of days and now it works flawlessly so props to Pentairs customer service
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