There are fairly straightforward methods for calculating the size (and type) pump that is neccessary for a particular hydroponics project. I suggest you check out the catalog from Aquatic Eco-Systems - they have great information on this subject and provide a wide variety of pumps.Though they deal primarily with aquaculture systems, they offer hydro products, and their tech support people are very familiar with the engineering.
Your variables will be: the size of system (how much water you are moving around), whether or not it will be running continuously, and the placement of the pump in relation to the system itself.