Two items come to mind. The first is that you need to be very careful in selecting an antenna because of the type of connector you'll need it to mate with on the WAP (and maybe the bridges, too). If I remember correctly, the WAP uses a "reverse TNC" connector, which has the opposite center connection from a standard TNC connector.
The second thing is that directional antennas for wireless networking are EXPENSIVE. If you're not careful, you'll spend more on the antenna than you did on the equipment, maybe all the equipment.