Many will recommend a 3 to 5x display tank turnover for your display tank, in your case thats 150 to 250 GPH at say 4 feet of headloss. I am familiar with the Eheim 1260 and it may be a little strong but you can always install a valve on the discharge side of the pump and throttle the return flow. I lean towards 5-7x the display volume depending on your sump size, baffle placement and the velocity through the sump before you encounter microbubble problems or an inefficient skimmer due to lack of detention time to do its work. If your sump is a 20G or larger you may be OK, if its smaller the velocity through the sump will be tremendous since it is only going to realistically hold about half its volume since you need to keep the level low enouigh for the skimmer and to maintain room for backflow in a power outage. 250+ GPH in a sump with 6-8 gallons of water is pretty fast versus 250 GPH in a sump containing 15-20 gallons of actual water.
You also need to match the return pump and overflow, either internal or hang on back so you don't outrun the overflow or starve it at the other extreme.
If the overflow and sump are sufficient I would recommend the Eheim, an OceanRunner 2500, Octopus of similar size or my new favorite for the last year has been the Water Blaster pumps by Octopus. Extremely quiet, very energy efficient and move gobs of water for the watts consumed versus most other pumps. A flow of say 250-350 GPH at 4 feet would be a good point on the pump curve to look for and compare the power consumption of whatever you are looking at to others, some like the Eheim and Water Blaster will cost a little more initially but will return it in power savings 24/7/365 and in their quality and long lifespan.
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