Re: Overheating question
If the water proves not to be circulating, the thermostat is the likely culprit. The water pump is unlikely, if the water pump impeller sheared off this would also prevent proper flow, but that is very unlikely. If it the water pump was leaking you would know that as well.
As you noted the engine need fluid added, it is possible you had an air pocket in the engine that hadn't been purged yet. This will also cause the engine to mysteriously over heat.
Suggest replacing the thermostat as they are inexpensive and most likely cause. Also if the thermostat is not working properly one of the large radiator hoses will be empty and not luke warm as it would be if the water was flowing properly. Ensure you "burp" all the air out of the system. Check the thermostat for binding and throw it in a cup of boiling water to confirm its failure after removing it.