All of the rules are actually freely and officially available online here https://2e.aonprd.com/Rules.aspx including [every monster officially released](https://2e.aonprd.com/Creatures.aspx), usually with a picture. You'd probably be better off buying at least the core rulebooks to actually read, though. Also, Dawn of the Frogs only has 4 pregenerated characters, so if you have more friends than that it might get a little complicated. So to actually answer your question, it depends on how much work you want to do and what your definition of "official materials" is.