That is early mileage for a Glow Plug job on a OM606. As you said, the headgasket probably was done in by the overheating.Make sure the head gets fully checked out (flatness & straightness) at a machine shop and gets checked for cracking too due to the overheat.If the head got warped too much, you will have to source a reconditioned head.Unlike a gasser, diesels do not take well to overheat conditions at all sometimes resulting in excessive oil consumption among other running problems.
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If your car is out of warranty, then this opens up some freedom for servicing and parts. There are several things you can do with your car out-of-warranty than when it's in warranty: