What is the best router extender to buy

Since I this is just a small town with not much going on around them, I don't think that there will be a problem with interference, but I thank you for mentioning that as i had not thought of that.  also there is a straight shot from the office to the fire station.  The only concern I have is that there will be some high-line wire running across the street and the line of direction will be just a little bit lower then the wires.  
          The line of direction will be about 15 feet above the ground
          I will be using an unlicensed band
          The only maintenance they will have is calling me when something goes wrong, so I want to get something that will work good and be reliable.

Thank you all for helping me with this.  I chose the EnGenius Technologies Long Range 11n 2.4GHz Wireless Bridge/Access Point (ENH202) and hopefully this will work.  I contacted the EnGenius and they assured me that it would work.  
Again, thanks for your help

The reason I like the 2-band device is that you can communicate between the radios on one band and communicate with the clients on the other band.  In that way there's not an added 50% data rate reduction.

Example:

With a repeater:

1) Send a data stream from the internet to the first radio (wired).
2) Transmit from the first radio on the one band.
3) Receive at the 2nd radio on the one band.
4) Transmit from the 2nd radio to a remote client on the one band.
Note: 3 and 4 can't be simultaneous if on the same radio/band.  They can only alternate.  Thus, 50% loss of data rate.  Receive some / transmit some / receive some / transmit some.... etc.
Also note that this is a one-way model.  Return data will do the same thing.  So, in the worst case with equal data rates the reduction is a factor of 4.  But this is rare.

With a dual-band set of radios:
1) Send a data stream from the internet to the first radio (wired).
2) Transmit from the first radio on band 1
3) Receive at the 2nd radio on band 1
4) Transmit from the 2nd radio on band 2 to a remote client.
Here, 3 and 4 can happen at the same time so the data stream isn't interrupted.
Again, this is a one-way model.   So, in the worst case with equal data rates the reduction is a factor of 2.  But this is rare.


Reality is a bit more complicated.  This is really just about the physics of the situations.