i love my quietone pump. it is actually very quiet and works either as submersible or inline, which is nice if heat is something you worry about, although doesnt seem to effect temp much as submersible for me.
size depends on how high it has to pump the water and how much water your overflow can deliver. quietone 1200 may be worth a look.
lowes actually have a good selection, or mine does, in the pond section. from 75gph up to something stupid like 1200gph. you need to figure out the 'head', how high the pump needs to push the water, and how much your overflow can deliver. if you have a manufactured one, the manufacturer or the instructions should tell you. then need to check the pumps and see how many GPH
it can do at that height. mine will do 570gph or something, proudly boasted on the box, but in reality, it only do that at 1 foot, progressivly less the higher you go.
a lot will have a graph on the box, or their website, showing you the GPH
at 1 foot intervals. i believe the quietone 1200 has an adjustable output, so you can slow it down if it pumps too fast. just dont get a pump that pumps more than your overflow can do. overflow will only go as fast as your pump.