Welcome to reef2reef !!
I went from my 20 gallon with HOB filter to a innovative Marine Nuvo 20 and I don’t regret making the change. It looks so much nicer and hides all the equipment. You can keep any type of coral but some good starters are zoas and lps coral. Like hammer, frogspawns , torches, and Duncan’s. Some cool fish are blennies, they have great personalities and are fun to watch.
Welcome to the forum! I definitely like my AIO (45 gallon) after piecing together tanks for most of my life. A good starter coral that moves in the flow as James said is Duncans. A Toadstool Leather looks good and is pretty forgiving. People always want to add Clownfish, but they can be aggressive so often good to add them later in stocking. A Blenny or Goby is a great start in a small tank. A Pink Streaked or Possum Wrasse would be a nice, peaceful fish that may help with small pests.
I also have a JBJ 45 gallon all in one (AIO). As a starter, I’d recommend you an AIO, so all the water stays in the tank on the stand rather than plumbing a sump underneath the tank. Check out my restart thread for all the cycling info and starting it up for a mixed reef. I’ll be uploading pics of corals tomorow.
As far as corals and fish. It’s all up to you. You can do a low flow seahorse/nonphotosynthetic (NPS) tank. You can do an SPS dominant tank for the porcelain/pastel look of a tank. Maybe all euphyllias of anemones for tons of movement in the tank.