Not really a problem, just like most cars when the cam belt is changed so should the tensioners and water pump at the same time.
In the 1.8 20VT engines, the water pumps failing was a common occurance, either the plastic impellor would break up into pieces (not good!) or it would come off it's bearing.
The solution was to use a metal impellor, however there has also been talk that the metal ones can also come off their bearing, and that VW/Audi use the plastic impellors for a reason.
If you buy a metal impellor, it's not an genuine part, so you will not get the 2 year guarantee from using an original part, to me, that's not worth the risk after thinking about it.