As far as "what makes a good air purifier," check the
CADR rating. The higher, the better. And more expensive, usually. Purifiers should have 3 CADR ratings - one for tobacco smoke; one for pollen; one for dust. You're probably most interested in the pollen rating.
Back when my wife smoked, we had some success with a Blue Air model 501. They're pretty quiet, too, if kept off the highest fan setting. Filters (there are 3 per machine, and they last one year) are around $75/set; double that if you want the carbon filters for cigarette smoke. I'm sure our model is obsolete now, but here's their website
Blueair - Air Purifiers
The website has a page for dealers near you.
Check Amazon, ebaY & craigslist, too. They'll run $300-$600, and should come with a new set of filters. I bought ours on ebaY a few years ago for $150 (it was a floor model in a showroom). That's a good place to find filters, too, when the time comes.