Speaking for myself, I won’t buy anything off of Amazon or eBay that I’m even mildly concerned about being counterfeit, and there are a lot of counterfeit Apple products out there.

I would suggest not taking the approach of “install all the things” and instead only install stuff as you need. That might mean you have a strong preference for Windows-style window management, and there are some utilities for that, or it might mean a specific email program you like, or productivity suite like MS Office. The very few things I think are worth installing are VLC (because it’s a much better video player) and the password manager of your choice (I use 1Password, and I would avoid LastPass or Dashlane, but there are many other good options). Otherwise don’t install stuff for the sake of it, you’ll avoid bloat and weird issues.