Building a network of trancievers at long distance
I am currently working on something similar and I am using silicon lab's Flex Gecko. They offer a range of Sub-Ghz modules with integrated ARM cortex mcu which would be perfect for what you want. 

They have a system called "Connect Stack" which allows you to create a network of wireless devices like the one you want in a few simple clicks within their IDE.



Every parameter is customizable and they also offer the RAIL API if you want to do something more custom than what the connect stack offers.

check it out : connect networking stack
(Their dev kits are also pretty awesome)