You can buy an adjustable thermo fan sensor setup at auto shops. Yes you can egt a 2nd hand one from an existing car, but mounting it may be difficult.

The adjustable ones have a temp probe that sits inside the top radiator hose. The kit comes with all the bits needed to get it to work.

I used a VN commo fan on my old patrol v8 conversion, with this sensor kit and it work a treat. If you are going the cheap, which you are, then this is something that you have to spend the cash on.

Then wire in the relay. I wired mine up some that I had a bypass switch on the dash. The unit would cut in/out as required, but i could also turn on the fan at any time I wanted. A couple of times on really hot days, 4wdriving, in low range working hard, it was cheap insurance to have the fan running before the big hill climb.