i've only replaced water pumps on these because they leak, not because of overheating. bmws, yes, because the impellers fall off, not so with mb. and mb pumps will leak while cold and not hot.

the no heat with defrost on means it was already low on coolant, could be just a weak radiator cap. get the cooling system pressure tested, without, then with the engine running. a stuck thermostat will impede flow to the radiator and also cause overheating.

if it did blow a head gasket, be prepared to timesert the block threads for the head bolts, they like to strip when heated badly enough. good luck, chuck.