I think it's a great idea. I'd use linux as my server since it's free and I'm familiar with it. You could also install a mail server and a web mail front end to make it easy to use. Or maybe a message board. There's a ton of free software for linux.

You'd want your wireless router, or the antenna connection outside the house to get decent range.

There's a similar concept called community wireless networks that people have been working on for years. There may be some software or ideas from that movement that you could use.