If you went with a 20 gallon tank you really don't need a skimmer as a weekly pwc will accomplish the same thing. For a powerhead I think you will be able to get by with a Koralia 1. For substrate, if you go sand get dry aragonite intsead of live sand because IMO live sand isn't "live" and many others will say the same. For salt brands everybody has a favorite. for lighting if you get a deluxe full hood or a strip-light with the tank, I think you can get by with that for fish and live rock. For a test kit a lot of people use an API saltwater master test kit. Since you're getting live rock you won't need a filter. Heater, a 100 watt would work great. For thermometers, I just use temp tape which is cheap but not the most accurate. Get a refractometer, much better than a hydrometer. I think that's about all I can remember.
