I have a 450 gallon tank with two overflows. Each over flow has just one 1.5 inch drain. I'm very impressed with how much water flows out of a 1.5 inch hole!
IMO you will be fine with one overflow if you go with a 1.5 inch drain or larger. Of course you need to plan your drain line plumbing for the least amount of bends and the shortest run of pipe possible to keep the water flowing as fast as possible to your sump. Try to position the sump (wet/dry tank) right under the drain so it's a short straight pipe going right into your pre-filter. Run your return line to the other side of the tank and you will have a nice even flow from one side of the tank to the other, very efficient!