You will probably need to change your substrate to aragonite sand or pool filter sand regardless. And you should cycle the tank (establish beneficial bacteria) using pure household ammonia or a raw shrimp before adding any livestock. IMO, this is where most people screw up when it comes to setting up any aquarium fresh or salt. If you do not have saltwater compatible test kits for ammonia, nitrite, pH, and nitrate, you also need to invest in those as well as a refractometer to measure salinity. (Hydrometers are not as accurate.) Be sure to test kits that use liquid reagents, not strips.
Unless you want to battle nuisance algae all the time, you will need a source of PURE freshwater. That means buying water from someone who can produce it such as your LFS or a grocery store or investing in an RODI system so you can make it yourself. You'll need to be prepared to do 10% water changes weekly.
What kind of equipment do you have for filtration, etc? Are you wanting to keep corals or anemones? Those guys require high intensity light which raises the cost considereably. Also, a 55 is too small for a dogface puffer or yellow tang in my opinion. Damselfish, while incredibly hardy, are extremely mean. (If you've kept mbuna before, they aren't much different.) I'd recommend looking on liveaquaria.com to get some livestock ideas. They have a spot where you can filter down livestock choices based on tank size.
If you are keeping any inverebrates (corals, shrimp, crabs, anemones, etc.) , I would also strongly advise you to invest in another small aquarium for quarantining new fish. The forums are littered with horror stories where aquarists bought a new fish, put it in the display and had it come down with ich and infect all the other fish. You cannot effectively treat most saltwater diseases with invertebrates. Your quarantine tank could be as simple as a 10 gallon plastic storage container with a heater and filter where you will isolate new fish for 4 to 6 weeks. (However, aquariums are easier to use becasue you can actually see the fish to figure out what, if anything is wrong with them.) Just like most other things with aquariums, bigger is better.