Replied this PM to a sis whose grandma suffers from lung infections.... (but I left out the qoo10 link...)
Hi there,
Just aim for the $50 coupon at 10AM everyday on qoo10, get it.
Then get the Samsung AX40 with 1 filter (comes with it) and 1 more extra filter.
Use it nightly whenever she sleeps. There is only 1 fan speed to switch it at, and that is in HIGH fan speed which I can 100% assure you that it isn't terribly noisy if you place it away from the bed. The Blueair and Honeywells are definitely going to be noisier (The Blueair 203 and Honeywell 18000 have a fair bit lower airflow CADR than the AX40...esp the Honeywell). The CADR airflow is 323 m3/hr, which is more than enough ACH/Air Change Per Hour for any HDB room. It consumes 27 watts at that setting.
Can switch on the aircon @ 26 or 27 deg C, RH won't dip too low at that setting, seriously. (the AX40 easily takes care of any secondary PM50/PM10 pollution, if there is any

Of coz, best is to nip the problem in the bud as it's always recommended to take care of the pollution source, in this case the aircon FCU is mostly the culprit as folks always forget/lazy to call for servicing which is $$$...or in some cases DIY clean even)
Leave a gap of approx. 5-10mm at the windows, as the "under-door" gap will always be there. This is to keep the CO2 levels below 2000ppm. This is not critical though, but thought i would recommend it. Presumably any VOC or formaldehyde levels also will be taken care of as well this way (assuming no new furnitures/reno).
Every year change the filter once. Replacement filters can be purchased on qoo10. Also can be purchased from Taobao (3rd party, very slight performance hit, in most cases not noticeable in real life).
There is no need for humidification, besides it increases mould issues.
I did not go into the details. But that's a done deal.