2D Sprite change event + Sprite Size

Hello, I'm a new Unity user with no Engine experience whatsoever. After looking into a dozen of tutorials I'm fairly certain my first goal will be archived pretty fast. Before I post my question I will first introduce my first goal: I'm trying to develop a 2D top-down zombie game as a (graphic)artist I don't see any trouble creating sprites nor do I see problems animating said sprites based on the tutorials I found, and that's my goal. My first question is based on these sprites which I could not find any answers too. One of the core gameplay elements will be based on multiple weapons, starting with none. After a weapon is in your possession the sprite need to change and again if you change to another weapon or back to having no weapon at all. Is there an easy solution for this?

Second question: I want to create this game on at least 720p, is this recommended?