Wind chill is kind of complicated, it's designed to estimate the "feel" of the effective temperature to human skin based on the ambient temperature and the wind speed. This involves two processes - one is that evaporation of liquids will suck the energy out of a system (it's an endothermic process) and makes it colder. Unless you've installed sweat glands in your tractor this is not likely to be a factor
However, anything that radiates heat will tend to build a "thermal boundary layer" around itself - this is true for warm-blooded critters like us people or for engines. This barrier allows the temperatures to increase faster, but a strong wind will blow it away. As an example of this, disconnect your radiator fan in the summer. The heat will build up because there's no "wind chill" to blow away the boundary layer.
In short, you're both right. Tractors are indeed affected by wind chill because a thermal boundary layer would allow the engine to reach a viable operating temperature much faster. However, the exact value of the wind chill is pretty meaningless when you're comparing something made from metal and plastic vs. something made of flesh and blood. You can make this point moot by blocking all air circulation around the tractor, wrapping it in moving blankets and stretch wrap will help it warm up faster but you'd look awful silly...
Of course, this only applies to something that's radiating heat, a tractor that's parked in the wind will be the same temperature as a tractor that's shielded from the wind. But don't discount fluid dynamics, strong winds will change the way air is introduced into the engine. In some cases it may force more air in like a supercharger, in others it will reduce the pressure and make the engine work harder for air.
I still think the best solution is to just avoid those temperatures altogether - I need to turn up the thermostat when I read these threads!
