Without starting the motor? Unlikely to be the cylinder head or head gasket if the coolant wasn’t full & not under pressure. Sounds like the block or a cylinder sleeve may be damaged. Not good news if so I’m afraid. Did you check the oil for coolant/water before pouring coolant into the radiator? It’s possible that what you have seen in the oil pan is condensation after it has sat for all that time? Fingers crossed. Also seems odd there was no coolant in the radiator. If the radiator & hoses are not leaking the coolant has leaked away somewhere. Hard to imagine anyone would have drained it & left it.