You should build a sump, with baffles, and a chamber big enough to fit whatever protein skimmer you're going to use. The whole reason to have baffles in a sump is for constant water level for a protein skimmer. If I were you, I'd run a skimmer on a SW tank.
This all depends on what you're gonna have in your SW tank. If it's a fish only small tank, I've seen people run canisters without a skimmer. If you're gonna have corals, and a large tank like you're talking about, then you probably want to have a skimmer.
Here's a discussion on skimmers.
http://www.thereeftank.com/forums/f6/do-i-need-a-protein-skimmer-154594.html
You need to really think this through-- a LOT.
A SW tank is expensive, and if you're going to spend some money to get one up and running, then you oughta have a solid plan, and be sure you'll be successful. And you oughta spend enough money for it to be successful.
Also, you plan an auto top off method. And you need to plan how you're going to make, and store, RO/DI water.
You have a lot of research to do.
