Using the MB blower regulator does sound like a good design. I wonder if the W124 blower regulator (124.820.27.10) might be better. It has a giant heatsink (resembles a Pentium IV heatsink), and MB engineers were smart to place it in the air-path of the blower so in a sense, it self-cooled itself!
Unfortunately, its a royal PITA to replace (they should have made an access port!) and also expensive ($274).
You did get the MB CTS values correct.
BTW- why would an op-amp be a better design?
From the "simple" circuit you posted, it sure looks like very few parts to go bad; and probably a very HIGH MTBF.
Perhaps you can make me TWO of these "simple" circuits OR TWO of the op-amp version.