All things aside, i would highly suggest checking out OPEN MESH.
The hardware is cheap, its "mesh-ready" to extend range where needed. It also has a management portal for your ap's & etc. I was planning on using this in a corporate environment but the back-haul is not encrypted for the mesh. As long as you dont care about that then no worries... I will note that they claim a user cannot connect to the back-haul network (only other open-mesh nodes). I do not know how true this is because i don't have the hardware to 'play' with but i would suspect for your specific application this would be perfect.
http://www.open-mesh.com/store/
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I will also note that the OpenMesh firmware will work on the Ubiquity NANOSTATION2 which has some insane output ;)
I know several people using this for WISP businesses. Even without an external antenna i was able to locate and connect to my nanostation2 at my office from about a mile away (on my side of the site i had a 15dbi parabolic, the other was just a stock nanostation2).
This would mean a slightly higher cost but with the added benefit of not having to have many nodes.. And to have the great range the ns2 has.
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-E
