Oh boy, something that I have a little experience with! I have the Red Sea 200. I bought it second hand about a year ago as my first saltwater aquarium. I think that the 42 gallon size has been a very good one, as a beginner. Performing a 10% water change is easy to do, it's a good breakpoint for stocking options, and it has a manageable footprint in my living room. The RedSea LED90, with all channels at 100%, gives PAR levels that allow for a mixed reef stocking (ie SPS, LPS, and softies/zoas if you create shadows. I think the Red Sea 300 is a bit of an awkward size because it's only 3' long - I don't think that's a suitable length for most wrasses or tangs, if that was something you would be interested in.
As for what I've done. I have an inkbird controller with two 150W Aquatop heaters; these have always been sufficient. I'm using a Jebao SOW-4 wavemaker for circulation/surface agitation and that has also been sufficient. I can't remember what my return pump is, but I don't think that I'd recommend it because it's pretty oversized. I'm using the stock ATO resevoir and the Tunze Osmolator 3152; it's a little fussy, but it works well enough when you set it up. I understock my tanks so I don't run a skimmer. I think the sump and cabinet that comes with the Red Sea 200 is a little cramped so I'm ok with not having it. I have a DIY lid and I'd recommend doing whatever you are comfortable with for sourcing your lid.
Right now, I have two ocellaris clowns, a tailspot blenny, and a royal gramma. With 50lbs of rock, they all have their own space and then more, so I haven't seen any territorial squabbling. I have eight trochus snails, four nassarius snails, and one blue leg hermit crab. With the host of copepods that I have, that has been plenty clean up crew to make it through the "uglies". I started getting corals about a month ago. I have one octospawn, one hammer, and one blastomussa. They all seem to be doing ok. I have a little hair algae in the tank that seems to looove the frag plugs, which the corals hate. A quick toothbrush scrub has been all that's needed for that though.
I think that it's been pretty great so far. I would recommend looking into other brands, but the volume and shape of the Redsea 200 has been what I've needed as a beginner.