I have a 1990 117.968 in a W126 SEC.
New radiator, viscous fan, electical fan. belts, hoses etc.
Car ran perfectly with temp sitting on 80 constantly.
Never burt oil.
AC guy installed a new AC Compressor, Dryer, AC belt & TX valve.
I then drove it down the Freeway (100km/hr on a 30C day), for 30kms and noticed blue smoke from the exhaust, and very high temp. Not sure for how long, as I was stupidly trying to figure out why the AC was not getting very cold.
Pulled over, engine siezed. Underside of Oil cap has melted.
It had dropped its coolant and oil level was very low. These both were fine the day before. Head gasket has blown (thats how it dropped ts coolant I believe).
Towed back to AC Guy. Car ran upon adding coolant and oil then. Radiator seemed to have pressure no viasable leaks. (haven't pressure tested the Coolant circuit yet).

Any ideas on the original cause?
Any views on overhauling the Engine after such high temps etc.