I think the first thing is remove the thermostat and drop it into boiling water and see if it opens. A quick test of the water pump is to remove the heater hoses and hold both of them shut with your thumb. Have someone start the engine. If the impeller is intact, you will feel pressure on the one coming from the pump. If that checks out, I would consider getting a leakdown tester. This will not only give you a good indication of the the rings and valves, but if the radiator bubbles, you know that the head needs to come off. I still have fears that the engine got hot before you owned it and the head either warped or cracked.
