Decide what kind of filtration system you want. A sump is the best option because maintenance is so much easier. There are ready to install mass produced sumps for that size but they are pricey. A less expensive option is a DIY sump made from a large aquarium, something bigger than a 55gallon would work for a 200 gallon I think. If you go with a sump than you should try to get a drilled tank that is ready for plumbing. I have a non drilled tank with a sump and it works great but the down side is that the overflow box is loud. You also want a skimmer. There are many options. I have an Octo in sump skimmer that is awesome. You can do canister filters but these are a pain because you have to break them down to clean them and change out the carbon which you need to do about every three weeks or so. With canisters, its also harder to control nutrient export.
Next you want to think about heating. For a tank that size you need 2 heaters that are rated for 150 gallon tanks. These should be attached to an external temperature controller. Mine was 45 bucks on
amazon so there are cheaper options than an expensive Apex system.
Next think about flow in the tank. This will depend on what you want to keep. Do you want high flow or low flow? Is not having obvious equipment in the display tank important to you? There are models with a low profile but these are very pricey. Mine are less expensive bulky powerheads and you can clearly see the cords in the tank. I hate them and will be upgrading when I have the funds. When buying these, pay attention to the flow rate in GPH (gallons per hour). By deciding if this is important to you, having a low profile in the tank, you could save yourself some money by just going ahead and paying the higher price now rather than paying the low price now and a year later upgrading and still paying the high price.
After this comes lighting and again that depends on the types of coral you want to keep. Plus there are three main types, T5 lights, LED and medal hylide, or hybrids of two out of the three. If you want a mixed reef or want to keep sps and more demanding corals or clams or anemones, do not skimp on this. I did at first and upgraded soon after when I realized my light was no good for keeping these and ended up spending more money. I should have just gone with the more expensive option in the first place.
Do you want a refugium? This is what
@btmedic04 was talking about when he mentioned do you want to go with the triton or Zeovit methods. Refugiums are part of the Triton method. They help keep nutrients like nitrate and phosphate low by growing the macro algae in a separate place so nuisance algae can't take hold in the tank. I have one and highly recommend it. My display tank is algae free and the tank is only 9 months old.
There are also other pieces to consider. ATO (auto top offs) allows you to not worry about checking the level of evaporation every day and fills the tank for you. I don't have one, I just check the level every morning and add what RO water I need. Then there are carbon and GFO reactors, UV sterilizers, and algae scrubbers if you want to go full one tech crazy. I don't because I don't feel the need with my tank but you might.
And there is other smaller stuff like a siphon for water changes, a pump to put new water back in, a barrel trash can or large Tupperware tote to make new water in.
TEST KITS!! Very important and again, don't skimp. API Master Saltwater Test Kit is fine for cycling the tank (get very familiar with this process before starting). But for more precise readings I recommend Red Sea for pH, calcium, magnesium, alkalinity, and Salifert for nitrate and phosphate. And that's not the best out there, its just what I could afford. There are also Henna testers which I hear are amazing but they are in the hundreds of dollars range. Under no circumstances should you do test strips, they are worthless. Also a refractometer. Again, you get what you pay for. Henna is better but pricey. Don't use one of those plastic things with a swing arm inside that floats to tell the salinity. I used one once and it was way off. A refractometer for testing salinity is really the way to go.
But first, before you buy any of this, YouTube "BRS 52 weeks of reefing." They are 52 videos that go over the setup and progression over a year of a 160 gallon reef tank. It helped me out tremendously. I also encourage you to read through some of the members build threads on this forum, especially those that have tanks similar in size to yours. That will help you decide what type of reef you really want in the end.
Hope this helps. Research and patience are the most important tools you will need for a successful reef. and it really doesn't have to cost an arm and a leg, just the arm will do.