I have heard rumours saying that the A/C - when being equipped with the diesel engine - has a minimum fuel system temperature? How big is this number? I heard ... something around 32°F?
"Something around"
-32°F; more like -37°C.
Basic Jet A, which you will use in your engine, has a "freezing" point of around -40°C (or F -- doesn't matter). You are required to keep a fuel system temp of 3°C higher than that to prevent freezing. Jets that fly often at higher altitudes use fuel with icing inhibitors that have freezing points of as low as -50°C. They also have fuel heaters that double as oil coolers -- the warm lube oil warms the fuel and is cooled at the same time.