The range will depend on what type of client adaptor you are using to connect back to the Omni with, and the surroundings.

A standard laptop with built in Wifi may get up to 300M or more line of sight. If you had clients radios with external directional antenna you could get up to 5km line of sight.

Each House would need to buy a small Directional antenna.

Directional antennas are used to focus the beam of radiated energy in a particular direction. Higher gain does not increase the overall power output but concentrates it in a beam giving more distance to the signal. Directional antennas come with different horizontal and vertical beamwidth's ranging from 4 to 180 degrees.
Go Wireless NZ Shop 802.11 Wifi Directional Antennas, Flat Panel, Yagi, Parabolic Grid, High Gain


The main thing you need to remember is that Wifi is two way communication, you will be able to see the signal for a long distance but the client adaptor has to be able to transmit back as well for it to work.

Most standard wifi devices are only 40mW and have 1 or 2dBi antenna.

The next thing is that you will need a coax cable to go between the AP and the antenna and it is best to keep it as short as possible. All outside connector joints must also be well sealed with coax seal tape or similar as moisture will kill the link.