First, are you sure there's not some environmental hazard that's causing these symptoms? Mold in the house, etc? I'm not any kind of relevant professional on that sort of thing (definitely not a doctor!), but it might be worth investigating and talking to your doctor about.

I personally own a few HEPA HPA300s from Honeywell ($200 each on amazon) for allergies. I buy the generic filter and prefilter refills from amazon, which end up costing $90 a year per purifier. If you've used other HEPA purifiers (not other kinds, especially the "filterless" ones!) and they haven't helped, I might suspect that your problem isn't something that can be solved by just an air purifier, or even helped by one.

Air purifiers are only potentially part of a strategy of managing allergies, depending on what you're allergic to. Seeing an allergist can help, and there are guides to the other things you can do at home to lessen your symptoms (changing hvac filters frequently and using an allergy one if safe for your system, pest control, mold remediation, washing bedding often, protecting mattresses, vacuuming and dusting, removing carpeting, etc. There are a lot of potential answers depending on your issue.)