I have a similar setup. I don't think you'll need an air purifier, but depending on the humidity (or lack) and temperature in the basement you may want to regulate those things so the air going into your room is in the correct ranges. I vent to a small basement window as well, and there's enough open window for fresh air to flow in as well.
Though you will eventually want a carbon filter so that you're neighbors don't think you've got a skunk breeding farm in your yard.