Don’t bother buying one without a HEPA filter.  Those that say “HEPA-type” aren’t good enough– it has to actually say HEPA.

The air purifiers with an “ionizing function” are probably not necessary– they can produce ozone, which is a possible lung irritant (according to Consumer Reports).

Check the square footing of the area you wish to clear!  I was a little bit sloppy about this when I bought my first air purifier and bought something on the smaller end.  Unfortunately, I just ended up having to spend more money to buy something more powerful in the end.