What makes you think it is the water pump? Unless it is frozen, squealing or corroded so bad on the inside that the turbine would not move water then it should be perfectly fine. You purchased a car at an auction with no maintenance history and it is overheating on you? Most of the time the auctioneer or seller will allow for you to have it inspected by a mechanic (he should be required to and if he dosen't then he probably was hiding something). You need to do a compression check on the engine and make sure it is ok and also change all the lubricants, engine, transmission, differentials and scrutenize everything for metal shavings, mixtures of fluids or any other abnormalities.