A skimmer is essential to have on a 55 especially if it's your first saltwater tank. You can get it later if you must, but it's not something I wouldn't put off for to long. Make it a priority.
sterilizer is completely optional. Most people never use them. I wouldn't take one if it was offered for free to use for anything but freshwater or quarantine though some people love em.
There are lots of different types of reactors. GFO
and calcium reactors are the most common and are very useful but you can live without them. I like GFO
reactors myself but you can wait and read up on them.
Depending of what you plan on doing with your tank these guidelines could change (ie. reef, fish only, FOWLR
, seahorse tank etc..). What kind of system do you plan on having?