You can get a hang on back overflow. Do not use a pump for the sump inlet. The point of the overflow is that it will constantly and automatically adjust its flow rate to match the output of the return pump. Using an electric motor for both would make it impossible to maintain sump and tank levels in balance. You would either empty your tank and overflow the sump, or vice versa.
overflow just needs to be primed, as it will work by siphon action.