I used to built out Intel 80C51 microcontrollers. But laying out the controller, power supply, clock/oscillator and wiring it all together was time consuming for each project. I moved away doing that due to the time overhead. I had several projects that would benefit from a microcontroller and reviewed the pre-fab options available (basic stamp, Arduino, Raspberry Pi, Beagle Bone, etc.). I felt the Arduino had the most add-on hardware modules available and (hoped for) a good support community. That is the route I chose. So far I have not been disappointed. I don't know if the others are better, but I do know the offering on the Arduino is pretty good.
With that said, I would suggest the Arduino UNO (
https://store.arduino.cc/usa/arduino-uno-rev3). Make sure to get a genuine one or you may start two steps behind (
https://www.arduino.cc/en/Products/Counterfeit). The Arduino UNO features a built-in USB programming/power port, a bunch of digital input/put pins, some built in ADCs, and a few PWM outputs. It does not have a true analog (DAC) output but I found this one
https://www.sparkfun.com/products/12918 and was easy to use. There are also several libraries available for different communication protocols (SPI, I2C, one-wire, and more). There are simple LCD displays readily available and libraries to match. Here is one store I have used for decades (wow am I old enough to say that?)
https://www.allelectronics.com/category/936/arduino-compatible/1.html. They have a fair amount of add-ons for it.
As for your greenhouse project, I would recommend one of two temperature sensors. Either the LM34 (deg F)/LM35 (deg C) and use the Arduino ADC to read the temperature or the DS18b20 and use the one-wire library. Both have plentiful examples on how get them working.
What is the output if you want to make the greenhouse warmer?
P.S. My apologies for selecting US based stores, but I think they will give you an idea of what is available and what to look for in France.
P.P.S. There is a subforum here under microcontrollers for the Arduino line, in addition to the Arduino.cc forum which is specifically for programming the Arduinos (no circuit talk over there).