Go to the flex-a-lite web site. They have a variable speed controller Part# 31165-VSC that is solid state. I've been told that this controller doesn't have the problems that the mechanical thermostat has.
I just went to a single speed as well. It worked great on low untill it got really hot outside. If you do want to use a thermostat (allows multiple speeds), get a solid state unit instead of the bymetal mechanical type. The solidstate will cost more but I've heard that they are more reliable.
GJ, don't forget the problems that I had with my fans while we were on the Pickle a couple of months ago!! Choose a good thermostat (not flex-a-lite 31147..it's junk), or I recommend wiring it to the ignition so its always on when the engine is running. If you have a bad thermostat like I do...