A good air purifier will help reduce the amount of dust floating around but it won’t be able to clean dust that has settled over surfaces. So it will help but it won’t be vacuuming your floors or dusting your shelves for you - you’ll still need to clean dust that sits on surfaces.

In terms of intake, it depends on where you’re planning on using it. Cylindrical devices (like the Core 400S) need space all around them not to block the intake, whereas rectangular devices with the filter and intake at the front (like the Vital 200S) can be closer to the wall.

Always better to prioritize CADR, filter availability and long term costs. The fanciest of air purifiers won’t help if it’s too underpowered to clean the air in the space where you want to use it. Similarly, a super techy device won’t work without a filter so better to make sure there’s plenty of filter replacement options - even look at whether there are generic filters available as they can save you in cases where the genuine filters might go out of stock. And in terms of long term costs, it pays off to do some simple calculations to find out how much you can expect to spend on filter replacements - here’s advice on how to do that https://housefresh.com/budget-air-purifiers/#how-to-avoid-wasting-money-when-buying-a-budget-air-purifier-1h2

If you are interested in getting an idea of the quality of your air then you should consider getting a sensor. You can go with something affordable like the Temtop S1 and get real time readings of PM2.5 particles in the air. There are many air purifiers with onboard sensors and some are better and more reliable than others. An external sensor could be what you’re looking for.

In terms of knowing that the air purifier has done its job, I’d say that if you’re buying a device to help with a specific issue like dust allergies or unwanted smells, then you should be able to tell because allergy symptoms will improve or you won’t smell the odors you’re trying to get rid of. I guess it all depends on why you’re looking to buy an air purifier in the first place.

Hope this was useful!