Dear Pierre,
Your project is very exciting - there is no good, cheap and reliable wireless sensors for a long distance on the market yet.
The task is clear and feasible for given time constraints.
The Adafruit Feather is a very good solution for the case. As the distance could be increased up to 20 km, solution becomes outstanding!
As Wi-Fi module gives the system outstanding flexibility - it can interract with user (ex. access code to open/close all doors in emergency) - it should be in a basic system. I propose to use ESP8266 Wi-Fi module as it is well spread, cheap and proved itself to be a highly reliable in everyday use.
I am professional programming engineer of AMX, Crestron, KNX, Arduino, Raspberry pi automation systems. I have more than 5 years experience in system construction. Currently I am an entrepreneur and develop wholecycle projects, from the idea to working system.
You may wonder, why the price for my realization is so low. I am newbie on freelancer.com and I need to rise my reputation promptly.
I propose two core ways to make this project:
1. (Best) To make sure the job is done perfect, I need the system to be tested in my office. In this case we separate job for two parts: preprogramming (12d max) + testing when I recieve all equipment (8 days)
2. (Good) Remote testing at your office. I write the programm (12d max) + we test system that you assemble by my design
3. (Worst) I just write the program. Testing is up to you.
Please contact me for details.