I think a good place to start is to think about the particular fish, corals, and other invertebrates you want in the tank. For example, if you really want certain fish in your tank and that's a big motivation for setting up a sw tank, start your planning around that fish or those few fish, as the types of fish you can put in a tank like a 75g or a 90g aren't necessarily the same as ones you could put in a somewhat larger tank. From there, you can work out equipment and such, building your system around what you plan to have in it, instead of trying to fit livestock you like into a system that may not have been planned out quite as well.