The system I had planned for my metro and never really got farther than see what kind of power output I got out of a wiper motor taken off a junk car. I was just going to use the wiper motor with a belt and sprockets driving the water pump. Using one of those adjustable radiator thermostats that has the probe on the long copper tube so I could run it through the water neck and into the block a good ways. With no thermostat and just using the probe set at something like 150 degrees seemed like it would work good enough.
But thinking about it it would be pretty easy to do a pwm output from an arduino or just a standard microcontroller for those comfortable with them to read the temperature and vary the speed from very slow to full speed. A standard temperature sensor could be wired in series with a resistor and fed 5V so the analog input on the controller could use it to determine the pwm output. Probably take like 5 minutes to write a program to do it. Instead of a mosfet and assorted wiring mess use a
IPS6021 (pdf link) so it is a 1 wire affair to control a 12V motor with logic level. The other neat thing would be to use 2 outputs and also control the fan speed based on how much power is needed so it takes less power to operate most of the time as well. The IPS6021 has a diagnostic pin output so if there was a problem with either the water pump motor or fan it could alert you with an indicator on the dashboard.