I would get a 600gph pump. It wont actually achieve 600gph, due to it having to overcome gravity as it pushes the water up the return line. If your pump is faster than your overflow, the sump will run dry. Some people will put a tee in the pumps return line with a ball valve to allow them to fine tune it by diverting a little water back into the sump. But why make things more complicated than they need to be?