I have a horrific crack around one of my exhaust manifold studs and it's still not leaked anything. I'd say headgasket. And unless the thermostat housing was leaking, a buggered thermostat wouldn't cause it to use water? Check the oil filler cap for mayo and give it a compression test. Leave it idling up to temp and have a good look for leaks. Check around the 4th runner on the inlet manifold to see if it's had the wrong inlet gasket fitted as that would cause a coolant leak too I reckon.