Things I've learned that you might find useful:

Lighting - lots of people (including myself) use the "Chinese lights". I don't know why everyone turns up their noses at them; with the huge number of people using them (because they are cheaper), I don't think the few horror stories about bad lights is really an issue. Get what you can afford and don't listen to all the hype.
Also with lighting - the cheapest ones (if you decide to go with LEDs) are the "manually dimmable" ones. Manually dimmable is pointless. Are you going to stand at your tank all day and dial up and down the intensity? No. So ultimately those lights become straight on/off lights per whatever timers you put them on. I'd recommend, if going with LEDs, to get a PROGRAMMABLE fixture, not a "dimmable" one. I also strongly recommend getting a fixture with at least a 1.5 to 1 ratio of blue lights to white lights. The whites will drown out the blues if they are 1:1.

I like a skimmer and in a 90 gallon I'd strongly recommend it. Many in-sump skimmers will work just fine; you don't need the most expensive thing on the market.

You DON'T need a reactor. They are nice, and I have one (I have two actually but I only have a nano running at the moment), but "needed"? no. That's an item I'd wait to find used once you have your tank set up and if you find a good deal then go for it, or you may decide you don't want one at all and that's fine too.

For your rock - keep in mind when buying your rock that eventually things will be growing on it. Your tank doesn't need to be stuffed full with rock or else it'll look cluttered once your corals start growing and just look a mess. Less is more. I probably only have 15 pounds of rock in my 29 biocube, if that. My filtration comes from, well, my filters. lol Rock is expensive so that's another thing you don't want to go overboard on (like the reactor) when you are just starting out and buying everything. It'll be a waste of money to get too much in the long run because you will be pulling it out to make room later on.

- the return pump is a big deal. Don't skimp on buying a good one of those. Where you can 'save' with the rock and not buying extra equipment, here is where you will 'spend'. That pump is responsible for really making your whole system flow. It has to pump upward several feet from your sump to your display tank AND have enough power left to spit out the returning water at a decent flow rate, so I'd take a 'go big or go home' approach to return pumps.

Deep or shallow sand beds or even bare bottom is personal preference, but for me, I think the idea of deep sand beds has run it's course and people are finally seeing them for the nitrate factories that they are. Again, others really like them though. It's something you'll have to research; I personally prefer a dusting or shallow bed strictly for aesthetics, but not because I think the sand gives any real benefit to the tank (in my mind, even though I have a shallow sand bed, the only "beneficial" sand bed is a non-existent one lol). Sand isn't too expensive either way.

And last, I'll just say don't buy livestock that you really have no interest in keeping, when you're first starting out. A lot of new-to-the-hobby folks just want to get something in their tank right away so they'll add some of the mushrooms that are really lacking in color, or star polyps (which do have color), or some xenia. While those are certainly "beginner" corals, they are also ugly and in most cases very invasive in your tank. Wait it out. Let your tank get settled and cycled and go for things that will still be easier but not that you are going to be scraping off your rocks later trying to get out of your tank because you just don't want them anymore. It's a waste of your money, and it's a pain to get those things completely eradicated from your tank later on. That also holds true of fish. If you want a school of damsels in your tank then go for it, but don't buy damsels just because you want a fish in your tank right away and otherwise really aren't going to want that fish down the line. Again, it's just ultimately money you could have waited and spent on something you actually want.

And that's all I can think of for now.