Everyone's got different ways of looking at things, but if it were me, I'd replace the water pump. It could very well be new, but it also could have been sitting for a very long time. Seals can dry out and bearings can get corroded. Since you're this far taken apart (and with water pumps being comparatively cheap parts), there's no way I wouldn't replace it now. Generally, they won't fail catastrophically.....but they can (i had a bearing go bad in one once, and it twisted the entire nose piece off of it) Therefore, since you're uncertain of its condition / age, I'd replace it. It's cheap insurance against getting stranded......or worse, not paying attention to the gauge and overheating it.
Just my $0.02
Just my $0.02

He's too quick.