90 gallon is 48" by 18", 24" high, right? Since it's pretty narrow, going for light bar will be more cost efficient.
Orphrek OR3 LED bar is great. Assuming you don't have high light coral like acro yet, you can start with one Reef Day Plus and one Blue Plus. That set you back for less than 400. Maybe have better price on black friday sale. Later down the road when you're ready, just add a couple more.
Another good long shape LED is Maxspect Razor X R5 150W LED. It's a bit over your budget, but not by a lot. You can also add two light bar to the side later if you need more light.
Good lights are not cheap, so think about not to invest all at once, but pick a solution that you can add onto later. If you get couple blackboxes, and a year later need to replace them for something better, the initial investment is all lost.
But either way you will want to build your rock a bit higher, probably up to 18", so that you don't need to buy big enough light to penetrate 24" depth while only covering 18" wide.
