Depends if you want to build your own or not [1]. If not, I think purpleair is the way to go (I use the same sensor as them but they have two I believe for redundancy). As for air purifier, I highly recommend Blueair. They’re expensive but was the only purifier I had that managed to pull the PM2.5 indoors to near 0 during this terrible wildfire season in California.

[1] https://www.powu3.com/airquality/