This subreddit supports the use of the cjdns networking protocol. This is because it implements encrypted IPv6. The cjdns whitepaper might answer more of your questions in regard to this.
This is a tough question, because ultimately we are trying to start from scratch. Right now, it's all being done wirelessly. For outdoor options, I suggest using a Ubiquiti NSM2/NSM5 or LocoM2/LocoM5 if you want to broadcast in a general direction. For a single link grab an airGrid M5. For omni directional, the most cost effective outdoor solution I know is the Ubiquiti Picostation. There maybe better choices, I just know from reading that Ubiquiti equipment seems to work.
This is not connected to the internet. This will run parallel to our current infrastructure, not on top of.
Apologize for late response. I have been trying to edit/update this sub to show updated information and links.