Pick up a used one
I have two filter queens the brown model and a triple crown. I picked them up a week ago from a lady for 100 cdn for the pair, which included tons of unused attachments, cones, manuals etc. She was selling on behalf of her parents. Both machines were well maintained and only required new bulbs.
There are tons of good deals on used models and I doubt the new ones filter any differently than the older models.
One thing that surprised me was I was doing my living room with my Miele S2 with the SB 228 powerhead. I thought it was was doing excellent job and when I vacuumed the living room with the brown model with the pn96 powerhead, it pulled up so much fine almost silt like material.... I was actually shocked. Not that the filter queen is better than the miele but its obviously still working well if its picking up that much that the Miele missed.
As for air purifiers, I think there are much better models out there. I have Austin air and Allenair models. Their filters contain 15 and 18lbs of carbon and hepa filters....