What Fan Controller

DCC would probably be the best option but the most expensive too. I had a thermostatic switch that threaded into the hole that is pluged on your thermostat housing and would turn the fan on at a certain temperature. Pretty basic setup, nothing fancy, but it was cheap.

You will need to wire it to a high amp relay though.

http://www.summitracing.com/parts/DER-16749/
 
What fan are you using? If it is the MKVII or Taurus fan you need to use a VSC controller that has soft ramp up vs. other controllers where the amps will spike upon startup.

+1 on Baskin's DCC controller. The best out there but expect to wait anywhere from 2-6 weeks for one. If you can't wait, then get the FAL 33054 from Summit. Do NOT cheap out on the controller. Spend the $100 and do it right the first time. It is far more expensive to do it right the second time or fix your car when you overheats from using the cheapo controller. I have a Taurus 2 speed fan and DCC waiting to go on my car.