What type of tank are you going to have? A reef tank with fish and corals? A fish only tank?
For a fish only tank you will need about 720 gallons flowing through your tank per hour.
For a reef tank you will need 1,440 to 1,800 gallons per hour flowing through your tank.
Sooo, lets say that you only have 300gph powerheads in your area. You will need at least 5 of them for a reef tank and at least two or three of them for a fish only tank.
The protein skimmer is pretty much mandatory.