You are getting asked what kind of livestock you want because some kinds of invertebrates and fish need more planning than others. Like anemones... they wander so you need to screen off any pumps or overflows that they could get torn up by. SPS coral seem to require higher flow, LPS could get blasted to death by it. A lot of soft corals will grow in less light. Certain fish need soft sand or coarse rubble at a certain depth to tunnel in. Some will knock over and throw rocks. Some must have a cave to retreat to.
Plumbing parts add up fast so if you are going used and spot a deal on a drilled tank with overflow and closed loop snap it up. Even if it's 150 bucks higher. Trust me
I just spent 100 and am only halfway there on plumbing my used tank, and I have to do the drilling myself. Should have spent more money...
You might want to take a trip to Red Sea and the other stores around seattle. Check out their coral and write down the names so you can make a list of your favorites. Maybe a top picks vs 2nd choice list in case they are not compatible. Check out what lighting they are using, do you like it? Is it needed for the coral on your A list?
Do the same with fish-make a must have list and a 2nd choice list, then research what they need, if they get along, will they eat your coral, eachother, snails, and/or shrimp.
Do your own research on the compatibiity using books and websites that are not selling anything. Keep in mind shop workers cannot be expected to know everything in the store, it's name, reproductive process, max size, eating habits, temper, behavior, compatibility etc etc. Most PhD biologists could not give that info accurately on all the stock in an average fish store. And some individual animals are jerks. And employees.
So take it on yourself, read a lot and save yourself some unpleasant and often costly surprises.
Don't believe the descriptions of animals on the big e-tailer sites, especially avoiding the critters labeled "expert care" or "no guarantee". What that usually means is nobody yet has figured it out. Not the pros at the public aquariums, not legions of optimistic amateurs who "just wanna try it". Some coral and fish don't make it period. You want to know which ones they are so you don't waste them and feel dumb too.
Good books I have enjoyed: Conscientious Marine Aquarist, Borneman's Aquarium Corals, Sprung has a pair of books on coral and inverts with pronunciations, Calfo's book on Coral Propagation (you know you will), Wilkerson has one on clownfish with good info on systems and feeding, and there is a Scott Michael series that is like an encyclopedia of fish. Not all of Michael's books are out yet. Your public library should have these so you don't have to drop 300 bucks on books
There are others too. I know some people don't like to read but at least check out the photos, and if something catches your eye just read about that. I think there are a ton of old books at the library too, watch the publication date. The technology of reef tanks changes yearly. There was a reason that people in the 70's had trouble keeping coral. Don't do what they did
Last but not least there are a lot of different ways to run a reef tank, successfully. Key is to not cherry pick the easiest bits of each method-skipping one step often means adding another. IE if you don't want a calcium reactor you have to add calcium. Decreasing the filtration means increasing the water changes. Not using actinics might mean using one very blue bulb IE Radium 20K. Not having auto topoff means toting jugs of water to the tank often daily. No chiller? AC and/or fan. There is always a tradeoff. Personally I don't mind the work so I add calcium, buffer, topoff. Some people want everything automated, computerized and probed.
But first you need to figure out which critters catch your eye and go from there.
Good luck, and have fun!
Kate