If you are new to boating, I strongly suggest taking a Boater Safety course. In some states that is required but regardless, it should be taken.
The recommended electronics depends to an extent on how you plan to use the boat and to a great extent, your budget. You might want to go to a Marine store like West Marine and look at various electronics to get an introduction to what is available. There are videos available online that discuss many topics relating to combo units that support maps, depth sounding, sonar for fish finding etc.
A VHF radio is pretty much a given requirement if you will be on open waters. Good radios can be had for under $200. You will need a good antenna, figure $100 and up for a decent model. You may also need installation, not too hard for the radio and antenna, a little more com[plex for the sonar/plotter.
Used electronics can be tempting and may suit you well and save a buck but keep in mind that multifunction units get dated and somewhat obsolete quite fast these days. VHF radios have a longer useful life.
You mentioned that the boat has safety equipment. I don't know what that includes bt you will need to make sure you have the Coast Guard mandated safety gear at a minimum.
Good luck on your boating journey!