Well most recommend starting with larger than a 20 if it is your first tank. Not that it can't be done, it will just be more difficult for many reasons just so you are aware.
As far as a protein skimmer, some do not use, and some do. It really depends what you want to do ie. reef, fowlr
, what filtration method you will be using....That skimmer is probably not to good but I do not know first hand. If you were going reef, a skimmer is a must IMO
Dry aragonite sand is much cheaper and will become live eventually when seeded with live rock.
Ask with more information as far as what you plan to do and how you plan to setup your tank and people will be able to give more specific advice.
Good luck and Welcome.