If I was doing this I would consider having target antennas on the repeater which point towards each of the houses. The spread between the two houses is to much for a single antenna I think.

The 1.5 km link is easy. For lowest cost grab a Linksys WRT54GL router and put openWRT on it. These are great as they take 12 volts at 1 amp. I have had a solar setup powering one of these with a car battery which gives a couple of days storage. You are going to need a big panel to provide enough juice and maybe want to also consider if you can use a small windmill to give a more reliable power supply.

This antenna should keep you going through good and bad weather.
WISP Router, 2.4ghz 24dbi Die Cast Antenna N-female, Client Antennas

Then I would consider buying two more WRT45GLs & antennas - one for each house to repeat the signal to the houses. These would be mounted with the antenna which receives the 1.5km link.

Antenna choice will depend on the terrain. At these distances a decent antenna should reach your linksys boxes inside the house. If this doesn't work you will need to put receivers on the roof of the houses.

Some people are going to say extra antenna and routers to repeat the signal on the hill is overkill but I think you are going to need them to be sure that you get a reliable signal.

Total cost should be around $800 for five routers and antennas excluding the windmill for the hill. This is a lot so you need to think if cable is an option.