You can fit a an electric fan just using a manual on/off switch and an electric fan off a scrap motor from the scrappies.
If you go this route make sure you put the fan on whatever side of the radiator it comes off the donor car.
If it's on the outside of the radiator on the donor, it'll blow the air through the radiator, If it's mounted on the inside of the radiator, it'll draw the air through the radiator. Either way it doesn't matter. Remember and wire the electrics the correct way, or the fan will operate in reverse!
If you go the manual switching way, you might be better to use a relay so as not to overload the wiring
After saying all that, the tdi engine usually runs happily without a fan, and would normally only beneift from one in really hot weather and when off roading.
Hope this helps