What should I consider when buying a respirator?
You have a few options. One that has been marketed to cyclists has been  the totobo mask which is from what I gather a re-usable N95 mask (although uncertified by any agency) with replaceable filter peices.  It's cheap to try, at only $25 or so.  

The Respro is another option.  It seems like a bit more  customizable, and looks like the neoprene would compress the mask against your face a bit better than the totobo, but it's probably really hot in the summer. These run about $60-$75 for a mask.

Here's a forum link which has a bit of a personal comparison from a few cyclists who use these masks in bangalore.
https://groups.google.com/forum/#!msg/bangalore-bikers/w2vgTWlsOH4/3KSlS0jOj0kJ