I just did my own timing belt (per Bart's very excellent instructions) and even though I wasn't having a problem with my water pump, I replaced it while I was in there anyway. After seeing how hard it was just to get to the dang thing I REALLY didn't want to have to go back in there until it's time to change the timing belt again in another 90k-100k miles. The cost to replace the water pump at a later date would probably be about the same as just changing the timing belt because you have to remove almost everything you would to do the belt just to get at the water pump. I would suggest doing it now to avoid the headache (and wallet ache) later.
I went with an Isuzu stock water pump because I figured if it lasted me almost 100k miles the first time then it must be good stuff. That's just me, though, and I'm sure others have used after market pumps with no problems.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless