You should first pressure test to make sure that you don't have a leak in the system before throwing parts at it. It could certainly be a bad water pump or it could be a cracked coolant tank. Look under the carpet in the trunk for coolant. If there is any then you have a bad tank that needs to be replaced. Once the system is water tight, you can then begin looking elsewhere.
It's a complicated system and quite big:
To improve performance, clean the area between the AC condenser and rads.
Good luck and let us know...