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1) Leaking. Loose shaft. If you can't see it leaking, remove the belt and see if there is "play" in the shaft. If not, it's probably OK. A water pump doesn't get "weak" with age. If the bearings and seals are good, and it's turning, it's pumping.
2) It is a fair amount of work. Before you finish, you'll swear that Chevy sent the water pump down the assembly line and built the car around it. It is NOT difficult, just a little involved.
3) My advice is to make several calls in your local area. Shooting from the hip, I'd guess $250.00 to $300.00. It's been a hundred years since I had anyone do it for me. Buy a pump for $60.00 to $100.00 and do it yourself.