The length of the tank is the biggest limiting factor. Though a 120 is a decent sized reef, it’s on the smaller side if you want to do a SA display with larger fish. That said, if the 48x24 footprint is the biggest you can fit, you still have lots of options.
I like to over-filter my tanks so take my suggestions with a grain of salt. Sumps are easiest to maintain but two canister filters will also work well. I like the idea of getting a filter that can filter the tank by itself, and then doubling up. Two fx4’s will do the trick just fine. Can always supplement with a HOB like a Aqua Clear 110. Hagen/Fluval is a solid brand and their more basic design paired with powerful motors really make them solid investments. If you want something nicer, eheim is a good brand but you’ll pay about 25-40% more for comparable filtering capability. Both brands are solid and it’s what I stick to.
two grown Oscar’s can work fine in this tank if they’re bonded and you can keep up with the water changes. Buy a small group of youngsters and keep the two that prove to be the most compatible. If you want something different, Tangledupincichlids has three different wild Oscar’s with the Crassipinnis being the most coveted of the wild forms avilable now. This Will also allow you to get a medium growing catfish or two. Something like a pair of spotted picture, or a marbled hoplo cat, or a single 4-line Pim just to name a few compatible, yet readily available options.
if you want to go smaller fish, your options open up quite a bit with your choice of stock playing a key role in how you choose to scape. Like in, you want to stick to the right proportions of fish size, scape, and tank dimensions. Personally, for a display, I would keep a group of medium growing (6-8”) fish with smaller dithers. This will allow you to scape the tank more.
If you go this route, consider getting fish for all three main water levels. A species that stays near the substrate, a mid water species, and a top water. with the Amazon having such a variety of offerings commonly found in the trade, your options are unlimited.
hit me up if you would like any other ideas or tips. Welcome back. No better side of the hobby than this one.