What size is your overflow rated for? That will determine what size pump to get, you want to have enough gph
to cover the overflow and the head loss (the distance the pump has to pump back into the tank is the head loss).
As far as submersible or external, well do you have enough room in your sump for a pump? If so it's probably easier to use an internal, but that can also raise the water temperature if the pump gets hot enough. So I guess, it's up to you.