You can buy a pre made sump or You can easily build a sump using an empty aquarium. I did mine with a 29 gallon aquarium and 1/4 inch acrylic for the baffles. I Cut the acrylic on my table saw and siliconed it in with aquarium safe silicone. For a 55 you could use a 20 gal long if it fits in your stand. There are tons of plans on lines and ***** is having their dollar a gallon sale this month. I used an eshopps 300 gph overflow box with my 55 to get the water to the sump. Worked well and less than $200.
Good skimmers are pricy but for a 55 you should be able to get a good one in your price range. I'm trying to decide what type I want for my 125.
Lights will depend on what you want to keep. For fish only not too expensive but SPS corals or anemones it can exceed your $500 cap.
Decide what you want to keep in your tank then do a ton of research to determine what they need as far as lighting.
That kind of goes for everything in a saltwater tank. Decide on the type of tank you would like and the inhabitants. Do your research, invest in a couple good books, then ask a lot of questions ( generally fish stores are bad places to get answers, not always but often).
Some of the questions you need to ask yourself are: Do the fish need a lot of swimming space, do you want coral, does that coral need a lot of light, will the fish you want eat that coral. Will the fish get along with each other. Will the fish be happy and healthy in the tank you have. How many fish do you want will that number work in your tank. Research is the key. You need to know what you want as well as what is feasible in the space you have. Lots of people want a pair of clown fish. They are easy and fine in a55 gallon as long as they are the same species but if you want an anemone for them it is a very different situation. You will need a mature tank with good water quality and the really good lights. If that is your eventual plan then you need to invest in the right kind of lights.
