Another thing to keep in mind is that saltwater holds less oxygen than freshwater, so you'll need some surface agitation and as much surface area for oxygen exchange as possible. For that reason, the tank stocking rule is roughly 1" per five gallons of water, but it also depends on the body mass of the fish you want. The 40 gallon breeder tanks have great dimensions for some creative aquascaping, and they provide good surface area.
You can create some nice marine plant biohabitats. Mangroves are pretty awesome, as are various sea grasses. Caulerpa macro algaes look nice, but be aware that unless you keep the lights on 24/7 (not a good idea with fish in the system), the caulerpa will go into a sexual stage and turn white and slimy. It's been reputed to wipe out a tank, not to mention that it's an awful mess.