need help

All four holes are 1 1/2" big? Thats weird. The drain is "usually" bigger than the return. There are two ways of picking a return pump(RP).
Some ppl pick a pump that delivers the same amount of GPH that the drain will handle and place some powerheads(PH) inside the display tank(DT) to create the flow that corals need.
The other way is to get a powerful RP to do both, return the water back to the DT and to create the flow needed. Thats the way I have my set-up. I have a Magdrive 18 with 3' of LocLine spray bars. You will have to decide which application you want to have. Luna

 

I'm not sure if you're looking for internal or external pumps. As far as internal goes you could most likely use a pair of mag 9.5 pumps, or one mag 18 (with a Y). I'm not well versed on external pumps, so someone else would have to pop in on that. You also need to decide what you're going to do about a skimmer (some come with pumps, many do not). Also, there are other factors involved, if you're going to add a lot of line (like have your sump some distance from your tank) you need to compensate for head loss. I hope this helps a little.