Do you need a variable speed controller?

It depends on what you'd like to accomplish, you really don't need anything other than a big relay to run your cooling fan. The results will be no worse, and in some ways better than using a conventional controller in that, although most controllers have a narrow margin between the on and off temperatures of typically ten degrees at the sensor, that's nowhere near the overall variance in the average engine temperature.

In a typical application, the turn-on and turn-off temperatures are set above the thermostat temperature in order to keep the fan from running continuously. This works fine, given that your cooling fan provides just enough airflow to cool your car, but consider the operation with a large fan in the dead of winter. The thermostat opens and, at several degrees higher, the fan turns on full speed. The coolant entering your engine is now at about fifty degrees Fahrenheit, and yes, your thermostat is still wide open, because the cold water hasn't made it there yet. When the coolant finally reaches the thermostat, where your sensor is located, the fan shuts off and the thermostat closes. So now, regardless of what your temperature gauge has read,  you've just temperature cycled your engine some 140 degrees over the span of about 30 seconds. The cycle then repeats until the car begins to move and forced air cools the radiator.

This is much worse than simply running the fan full speed continuously, in that a constant running of the fan will at least allow the thermostat to settle in order to restrict the coolant flow. The nearly closed thermostat will also promote coolant flow through the water pump bypass and, in turn, warm the incoming coolant. You could locate the sensor at the engine inlet, but realize that regardless of what you've been told, the cycle period of the fan with these controllers is not determined by the span of on and off temperatures, but rather the time that it takes for the cool water to arrive from the point of being cooled to the point where the temperature is sensed, so your fan will cycle on and off every few seconds until the relay or fan motor fails. With regard to running the fan continuously, electric cooling fans are designed to run only when the car is at a standstill, so running it continuously is not a great option either, as you will soon exceed the service life of the fan motor.

Is temperature cycling damaging? A good place to start looking for that answer is at the cars that you've had that eventually gave up the ghost. Was their demise due to oil consumption, a connecting rod through the side of the block, or was it a head gasket or the end result of overheating due to a radiator failure ? Then consider the fact that what you've seen isn't at all uncommon, in fact a cooling system failure is statistically the most likely failure to leave you stranded on a given day. There is also good reason for these failures in that the materials used in both the head gaskets and radiator are prone to work hardening, With a cast iron block, copper composite head gaskets, aluminum heads, and hardened steel attaching bolts having far different expansion coefficients, it should be no surprise that the gaskets will have a limited cycle life. Given that your radiator cycles from + 14 psi to -3 psi each and every time that your fan cycles on and off, it makes sense to avoid drastic temperature variations in order to prolong its lifespan.

So when exactly are these failures going to occur? The typical radiator life is about 70,000 miles if it's taken care of, while a head gasket will typically last about 100,000 miles, but with a fairly wide variance in the point of failure, so increasing the life of your cooling system lowers your odds of failure at a given mileage by the same amount. If you plan to keep your car anywhere near these two points of average failure, given that today's price of installing  a head gasket is between 1.000 and 1,500 dollars, then the decision to go with a variable rate cooling system is pretty straight forward. $110 invested today will eventually save you $1,500 in today's money. If not, then it's a matter of lowering your odds of failure.

The DCC variable rate current controller will not only substantially reduce the incidence of the above mentioned failures, it will also increase the life of your cooling fan while reducing the noise level and current consumption significantly.