Ok, this may be the real way to do this:
http://www.telcom-semi.com/download/lit/pline/analog/thermal/dcfan/21450a.pdf

8 pin chip + one transistor to drive the current.
The chip has internal temperature sensing with ~ 1-3 degrees accuracy (good enough), and PWM output to drive the fan. You order the chip with pre-determined tempL and tempH parameters, which is a bit restrictive, but it will give us the ability to have a fan that is off until eg 35 degrees, and then ramps up to max at eg 50 degrees.