Just fit a thermostat and use the existing electric fans, they are almost never used for the aircon so are sat there doing sweet FA. they are certainly powerful enough for the diesel and probably OK for the petrol. Stat goes across the switch on the aircon dryer, I can sort out the switch number if you are interested

If you buy fans, always go for a twin set up, electric fans do fail and with one big one you are in the sh*t if it gives up.