I've spent many hours navigating the air purifier market and it is one of the most opaque and customer-unfriendly ones I have ever seen. Beyond the advice given in the article, it's also important to check what the filter exactly is. Some filters are washable, but it often means that they are not as thorough as HEPA filters, which is a big deal if your goal is to avoid the dreaded PM2.5 that are as far as I can tell the main threat to health and the pursuit of happiness.

A more general problem is that there is a conceptual contradiction between low noise and high filtering. It will be difficult for a silent filter to be quick at filtering your room.

The good news is that even a relatively crappy, low CADR setup can still filter a bedroom over time so long as it remains closed. You will however be paying for replacements more often and it might not be worth it if you have to ventilate daily and the filter takes many hours to give you a clean environment.

I like the work that Smart Air China is doing. I am not affiliated with them, but they have essentially done and made available the same sort of research as the one highlighted in the article and have long promoted the no bullshit use of a simple box fan with filter combo for lower income and highly polluted regions.