There are few things that I would like to clarify -
You want to go for a directional antenna but you state that there is some obstruction between the AP and the wireless clients [laptops]. This suggests that you are not configuring this unit as a bridge for a point to point connectivity.
If you want to use it as an AP for providing connectivity to wireless clients, using a directional antenna won't be a good idea because the horizontal beamwidth is low for directional antennas typically 10 to 30 degrees.
Although you can have a look at this antenna -
This gives you 55degree horizontal beamwidh.
Typically you should be looking for omnidirectional antenna is the AP is going to be at the center and the clients are going to roam around the AP in all 360 degrees. Looking at the range you have mentioned you can look at -
AIR-ANT1728, 5.2 dBi, 360Â°H, 36Â°V beamwidth.
Approximate Indoor Range at 54 Mbps
114 ft (35 m)
Approximate Indoor Range at 6 Mbps
379 ft (116 m)
Or else if connectivity is not required in all directions then you can look for some patch antennas, the horizantal beamwidth is greater than the directional antennas typically 30 - 55 degrees.