Don't forget to take into account head loss! In other words, if you get a pump rated at 300gph and set it in the sump on the floor with a return four feet above, there won't be 300gph coming out because the pump has to overcome the back pressure. Most of this information can be found on the pump packaging.
Whatever the flow rate on the pump is at the head height you have, you want the overflow rated that or higher so the pump doesn't drain the sump faster than the overflow can drain the tank and refill the sump.
having a valve on your pump is a good idea anyway IMO
if you need to remove the sump for maintenance or something, and that can keep you from having a flood or otherwise making a big mess.