100k miles = ~10 years for normal use, so use the decade mark as a rule of thumb if you are well below normal mileage (that's the outcome of the debate on the 928 forums - age is as important than mileage, if not more so, for parts).

That said, there are two schools of thought on the water pump... some say replace it while you are there. Others say it's foolish to replace a proven-to-be-perfectly-good part with an one with unknown reliability. More than one person has replaced a water pump when they did their t-belt and had it go bad in the first couple of weeks.

Replacing the tensioner is a very good idea, though, IMHO - it's seen some wear and tear after this much time.