I have used the sensor you speak about before however that grounds to pull in a relay. I'm not willing to experiment with the "probe" wire to see if that is how it actually works. I think it does not work that way since it has no ground on it.
I think it merely senses the temperature. If I went your way I would have to redo the control system. Not impossible just a lot more work
As far as suggestions for fan units. Well the price of an assembled unit has gone astronomical. There is this for example
http://www.summitracing.com/parts/flx-280
For over 6 bills.
With regard to electric fans here's my take. First do not pin the fan to the radiator where the pins go through the radiator. The vibration will eventually loosen the fan and rub a hole in the radiator. If you can build a fan housing and build it with fan motors, fan blades and a control kit like Orange mentioned would be the least expensive way to go.
If you don't have plans for the housing, try to get people to let you see theirs for measurements. Mine has 2 fans and most do for our trucks. The design ultimately rests on the width of the radiator.
Good luck