sptt, it's odd the low coolant level warning light didn't illuminate during this overheating/loss of coolant incident. If your friend isn't a MB-trained tech I would have an indie who is evaluate the condition of your engine, and propose the most sensible and cost effective repair method. If the indie has a borescope he can remove the spark plugs and inspect the cylinder walls to see if any cylinder walls or pistons are toast. Pulling the head will reveal head gasket failure or valve problems.

As mentioned any blockage of coolant passages is undesirable so that's a major consideration befoe deciding to repair your existing engine. On the other hand buying a used engine has a downside since you probably won't know its service history, and it may require major repairs in short order.

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Fred Hoelzle