Replacing the water pump is part of the 100k service along with doing the timing belt and various seals. Change the water pump while you have everything apart for some peace of mind. Water pumps can start showing signs of leakage at around 120-150k miles which is in-between the 100k timing belt change intervals. Don't just repalce the timing belt only, and find out that you need to tear everything back down again when the pump starts to leak shortly after.

This part is $230 retail at the dealership. We sell the exact same thing here for much less. Includes front and rear housings, o-rings, and gaskets needed for a full water pump replacement.

This pump works for the following:
2JZ-GE VVT-i Engine
2001-2005 Lexus IS300
1998-2005 Lexus GS300