If you are curious to play more with plugins the Avid Complete Plugin bundle may be a place to start and rentable/subscribable for relatively low cost (or is included in some Pro Tools subscriptions). Otherwise if you ask here for recommendations for what you want to do folks can suggest plugins or bundles. Be prepared for some sticker shock
Oh and used plugins... that's a very long story. If Avid plugin you may be better with the Complete Plugin Bundle. For other vendors it exactly depends on how they do licensing, and their transfer policy/process. You also need to understand what plugin version any licenses you would get license you up to, again all vendor/plugin dependent. And for compatibility with your Pro Tools that plugin version(s) needs to be in AAX-64 format. Some advertised here are RTAS and/or TDM.