I suggest you google Premiere Pro Plugins...then ask for opinions on what you find that you require.
I was one of those mis-guided individuals who compensated for my lack of experience by collecting 3rd party plug-ins, especially when they went on sale at irresistible discounts. Thinking that they may come in handy someday, I wasted money on a bunch of them I never use.
However, there are some I use on almost every PPro or AE project. Among those are plug-ins from Red Gaint for color, keying, and particles and re:Vision for speed control, motion blur, noise, etc. I tend to apply more of these in AE rather than in PPro.
For audio, I rely heavily on plug-ins from iZotope.
In AE, I also make a lot of use of the plug-ins from Video Copilot for lens blur and 3D.
If I had it to do all over again, I would have learned PPro and AE from top to bottom before buying a single plug-in. These days, when I want to apply a transition or effect to my footage that is not natively supported by PPro, AE usually gives me what I want.