I have used these controllers on several cars I have converted onto an electric fan.
http://www.cbsonline.co.uk/adjustabl...stat-763-p.asp
The sensor can be mounted externally; no need to open the coolant system and risk a leak. Just fix the sensor to the radiator or a pipe and adjust the knob to make the fan start at the temeperature you wish.
Once fitted, I suggest putting something over the sensor to insulate it. That way it will reflect the temp of the coolant, not the air that might be passing over it.
John