a) The AirPort Base Station is already effectively a firewall (unless you are looking to firewall the WiFi from the connected clients). Short of a Cisco product (or equivalent) you are not going to find better firewalls, certainly not with a print server and not in the "low cost" realm.
b) If you have one of the newer AirportExtreme or the AirportExpress base stations, then they already have a USB connection to allow them to function as a printer server.