I tore down my couple of saltwater tanks and one of them was 20g. So many ways to approach but it all comes down on what you want to achieve. With 20g you can either set it up with HOB and a powerhead, or have a sump, you need to drill it to have overflow box, it's the preferred and efficient method. I drilled both of my tanks, very easy. With lighting, unless the Planted plus has at least 10k and blues, or no corals, then you need to replace that. I've used lightings from Kessil, t5s, but I eneded up using Reefdreeders Led, they're great for the money and gaining popularity, very nice fixture. You need to be careful how much lighting you put, algae can be a real nuisance in saltwater tanks. Reef breeders led, you can adjust the intensity. For live rocks and sand, you can start with dead live rocks, they will become live rocks after a while same thing with sand. You just need to buy couple of live rocks to start the coralline algae, the pinkish algae you see in saltwater tanks. This way you don't need pay to much for rocks. You still need to cycle the tank before placing corals and livestocks, these are just the basics. You need your normal test kits plus test kits for saltwater, calcium/alk/mag, a must if you will have corals, otherwise no. With saltwater, you need lots of research, patience and take your time picking the right equipment, it is expensive.
For 20g, you don't really need a protein skimmer, you can get by with weekly water change. But if you have a sump, you can place a skimmer in the sump, either in tank or HOB skimmer. You can go to reef central reef site for more info.