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I think openwrt on the main unit (a good router) + nodemcu based sensor units would work well.

Openwrt is powerful enough to take care of logging, connecting to the internet etc, and the sensor units can form a pseudo mesh using their wifi capabilities.

Ideally the base should be from a project that handles base system upgrades, so that consumers don't have to trust that the vendor is building security updates in a timely manner.

Last I checked, OpenWRT did not do security updates, and it's up to the user to recompile everything if they want newer versions.

Canonical have been doing work on this with Ubuntu Core http://www.ubuntu.com/internet-of-things

Imagine if everything ran a Plan9-like OS.

https://en.wikipedia.org/wiki/Inferno_(operating_system)

That gets you:

1. Communications between devices are encrypted by the OS.

2. Access to the devices are handled with simple file-permissions.

3. Feature-providing utilities can be run either locally (i.e. on the device's CPU) or remotely (on some hub-CPU.)