Your X3 probably had the M54/M56 engine that along with all BMW 24 valve 6 cylinder engines have a water pump prompt to failure in the 40-60k miles range. OTOH older 6 cyl and V8s have a pump that last much longer.
blue dragon, I am sure that your SA has a very limited knowledge to the diesels, and she just assumed it has the same type of pump as M54/M56 engines. Considering that the diesel rarely exceeds 3000 rpm, I would not consider changing my pump until it fails or at a time of a substantial overhaul of the engine. That said, it is your car, and my assumption may be as wrong as of the your SA.