Usually the only pump needed for an overflow is a small aqua lifter ($15) that is in place for any power outages. The overflow will break it's siphon when the power goes out and when the power comes back on the aqua lifter will suck the air out and start the overflow process again.
A return pump is required in the sump to pump the water back to the display. The size of the return pump is based on what you are doing with the sump and how big the sump is. This could be a long conversation, so please let us know what your thoughts are about the sump (only extra water capacity and equipment area, refugium for macro algea/copepod development, etc.).
There are lots of knowledgible folks here (and threads) that talk about this. Check the Archives & FAQ section.