Sorry for the delay, I've been working on.
Thanks a lot to everyone! I've chosen to use a relay specially for arduino, so I will not need a module (easier)
( this one https://www.google.be/search?q=relais+pour+arduino&client=firefox-b-ab&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi1gtiY16zPAhVMHxoKHRf9DQMQ_AUICCgB&biw=1536&bih=731#imgrc=fUrt6ib9TFD7DM%3A)
I will not use a step down regulator because i will just use the 12V to power the arduino directly.
Now I'm working on the temperature sensor, this one :
http://www.burton2cvparts.com/capteur-température-huile-p-6272.html?language=fr
However, I'm still trying to see how it works exactly and if I can use it for the arduino. If someone knows or has any ideas, they are most welcome.
All the best!