If you have COPD or other serious lung issues, don't get just one HEPA filter. These machines are rated to filter the air in an area that's certain number of square feet, based on how many CFM of air they move. Get
at least enough of them that your whole living space is covered, and deploy them throughout the entire dwelling. You may want more individual machines so you can run them on a lower fan setting, which produces less noise. Assuming availability of outlets, you can also go for a larger number of smaller machines, each with a lower CFM rating, which should also reduce the noise level somewhat.
I personally use Winix machines. I don't think it matters much in terms of efficacy which brand you use, so, consider the cost of replacement filters here. The machines I bought were around $200 each, and worth every penny, IMHO.
My machines are both made for environments with pets, so they have a pre-filter to catch large things like dog fur, and they also have an activated carbon filter to pull odors out of the air. The carbon filter is quite effective. Unfortunately, I was able to test this first-hand, by observing that I could no longer smell dog poop after about 10 minutes of the machine running in the room said poop was in.
I wouldn't worry about silly features like accompanying smartphone apps, or any kind of "ion technology." IMO, that stuff is mostly marketing fluff, and just adds complexity to what should basically be a small fan with a filtered intake.
I've found my machines to be very effective in cleaning the air inside my apartment. I've been lucky enough so far to not have any issues from the Bay Area smoke, despite having asthma and allergies. I credit that to my HEPA filter machines, as well as wearing an N95 mask outside whenever the AQI is too high.
As an aside, I once put one of my machines (each rated for around 300 sq. ft., IIRC) inside my bathroom, which is no more than about 30 sq. ft., for a while. Eventually, the air in there smelled so clean, it literally took me back to the days when I used to regularly work in a class 1,000 (ISO 3) electronics clean room. "Class 1,000" means the air inside is supposed to contain no more than 10^3 particle >= 0.1 µm in diameter per m^3 of air. Once you've smelled the air inside a decently high level clean room, you'll recognize it when you come across it again. It smells like a glorious whole lot of nothing, which makes my allergies happy. :-). For reference, ordinary, unfiltered, not-apparently-dusty air typically has about 10^6 particles >=0.1 µm per m^3.
