What's involved in changing the water pump on this car? And where a good (cheap) place to buy an OE pump?

Where you live would depend on who around you has the best price if all fails go to thebenzbin.com... As far as removing the pump set aside atleast 4 hours minimum... remove the radiator, then the fan belts & pullies and any thing else that is virtually in the way of sliding the pump straight forward...