For uncured product (Bondo and poylester resin) you want an organic rated resperator. I get mine from Home Depot. Depending how often you work with it it is recommended to change the cartiges about once a month. I also typically by a new mask about once a year.
Since we are on safety, you should wear gloves while you are using this stuff. Read the safety sheet for Bondo, but most likely it will say to wear latex gloves, NOT nitrile.