Do you already have a skimmer? If so is it a in sump or external? Are you going ot use a filter like bio balls or rubble rock? Thats seems like alot of stuff to fit into a 18X15 space. Skimers take up alot of room. 1" drain and return may be a little over kill to. My 70 has a 1" drain and return. A drain i would do a 3/4 or maybe 1" the return i would do 1/2. After you get the sock and the skimer, plus if you do filter media. Theres not alot of room for a fuge and a return pump.
Are you going to build the sump? or have someone make it for you?
Joe.
70g Reef/ 250w 14k + atinics