I just looked at the instructions for this controller:
http://www.speedwaymotors.com/Spal-Pulse-Width-Modulated-Fan-Controller,35700.html

and I don't believe it will work with the mercedes fan. From the wiring diagram, all of the current for the fan goes through the controller. Apparently they're using a 30A mosfet, or something else to switch the current, leading to the 30A limit.

Looking at the first fan thread, there are two heavy wires going into the fan, and another smaller trigger wire. The trigger wire is where the PWM to control the speed would go, while the other two wires carry the heavy current. (I'm SWAGing that this is true)

In other words, ideally, the controller itself is not switching the current for the fan; it's just telling the fan to turn on and off really fast. If the signal is on for 50% of the time, the fan will run at (roughly) half speed. Same thing for 25% or 75% input on the trigger wire.

I just looked at a few other controllers, and they all seem to work like the SPAL one above; just a relay to turn on/off the fan at different temperatures.

If we can't find a controller that will do that, I could whip out a design that would probably cost $40-$50 that would do just what we need; a simple PWM output corresponding to the engine temperature.