As I have ascertained myself, I think the buyers guide should say: don't buy a water cooled one.
I have a water cooled BIS from 1989, and it costs a serious amount of money to keep it on the road, because I had to change about everything inside the engine area (except the engine itself).
The car had been on a field for 14 years when I decided to buy it, so I guess that also has something to do with it.
Are there other water cooled drivers who can contradict or confirm if the water cooled is really worse then the air cooled?