I was talking to an Oreck salesdude about 10 years ago after purchasing some vacuum cleaner bags.
I noticed they also had air purifiers and Mrs. Toe had been having a few allergy-type reactions in our home at the time, so I asked him about them.
I don't remember everything he said, but the way the Oreck machines do what they do impressed me and I decided if we ever had to get an air purifier, it would be one of theirs.
I think one of its features was that you don't just buy new filters several times a year... the ones he showed me, you pulled out a tray and dumped out the dust/pollen, etc. and the filter system was removable and washable.
I'd recommend going to an Oreck store and asking a lot of questions... looks like Amazon sells them, too.
https://www.amazon.com/s/ref=nb_sb_ss_i_2_5?url=search-alias%3Dgarden&field-keywords=oreck+air+purifier&sprefix=oreck%2Cgarden%2C317&crid=C25R2YDZ7FHU&rh=n%3A1055398%2Ck%3Aoreck+air+purifier