Long range (12 km) wifi link via high site

I want to set up a wifi link to give my parents a communications link on the farm where they live. Their house is only about 12 km from the town, but they're in a valley with no phone or cellphone reception. My plan is to get an ADSL point in town, and then put up an access point on a high site with LOS to the town and their house. My problem is: knowledge. I need advice on how to do this the simplest and cheapest way. I've set up a blog that documents my project, please have a look and please leave comments and advice if you have any. The link is :
Diywifilink's Blog

There's a map and a better explanation on there, as well as the information and advice I've received up to now.

Thanks!