Multipoint Antennas

I'm looking to connect three farm houses using Buffalo .11g gear
(using WDS) to share data and share a cable internet connection. The
distances of the paths are 1.25 miles each. I have LOS for the paths.
I plan to use relatively short (<15') lengths of LMR-400 for the
outdoor antennas. I want the best signal (who doesn't?) so I can get
the most from the 11g, realizing that WDS will cut my bandwidth in
half. I figured I'd use a router with a directional antenna at the
house with the cable available, an AP with an omni at the middle house
(it would also have clients connecting wirelessly), and either an AP
(for multiple computers) or an ethernet converter (for a single
computer) with a directional antenna at the third house:

cable/router/directional->-------<AP/omni>-
-
about 125 -
degrees -
-<AP/directional

My questions:
What is the proper gain needed for each antenna?
Can the gain be too great?
Do the antenna gains need to be "balanced" some way?
Should I consider a sector antenna for the middle AP?
What grounding, if any, is needed? The antennas will be at or below
the roof lines of the houses.
What length of LMR-400 could I use without losing too much signal?
LOS between the last two houses may be encrouched upon by a rotating
irrigation boom (4" aluminum pipe plus some smaller pipes). Will this
completely destroy the LOS for this segment?

TIA,
R