A chain stay protector keep the chain from marring the finish of the chain stay. The Aventon likely has a piece of helicopter tape on the stay to protect it. There are fancier ones available. If you ride on a lot of rough roads or off road you will get chain slap. but otherwise you don't really need one.
As for stuff you should get in advance: helmet, floor pump, and things to fix a flat (tire levers, spare tube, patch kit, and emergency pump or CO2 inflator). Anything else you can wait and see what you want/need.