Engine could just be stuck from sitting. That would be relatively easy to deal with.
I recently looked at a 280SE 4.5 that was being revived at a local repair shop after sitting in someone's garage for 20 years. Most expensive part needed was a new high-pressure fuel pump (electronic fuel injection on these V8s)
The radiator coolant looked brand new (like in my DKW)

Two years ago, I revived a neighbor's '72 220, that had been sitting in a garage for 18 years. That engine wasn't seized either, but it did need a waterpump, alternator, belts, carb rebuilt, motor mounts, one valve adjuster, fuel hoses & filter, all brake hoses, rear calipers and rotors (but the fronts were OK!) neutral switch, shifter bushings, heater blower motor and intake/exhaust manifold gaskets.

Worst case - If the coolant in that 220SE was in a neglected state when the car was parked, the head/gasket can corrode, dumping old tired coolant into the crank case and turning the engine internals into a rusty lump!

Happy Motoring, Mark

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DrDKW