Since you are adding a sump I take it your tank is not pre-drilled. On that assumption you are going to either need to buy an overflow or build your own. If you buy one, then your last question should be answered because you should be able to buy an overflow rated for a specific GPH
(gallon per hour). If you build your own then the only way to know is to test it (by having it drain a known quantity of water for a known amount of time then do the math).
As to building the sump, more things are needed to be known. Are you going to get an aquarium and build your own? Are you going to buy a pre-made? What are your plans (such as wanting a fuge or rubble?)