You can likely fix the coolant leak in the head gasket using Barr's Leaks Radiator sealant. I used a small amount in my '97 E 300 D, and it has kept it sealed for about 7 years, with a couple small additions of the sealant. I put the sealant into a bottle of coolant and shook it up. After the large pieces settled out, (about a minute) I poured it into the coolant reservour leaving the dregs in the jar. You should drive the car for a half hour or more to help the sealant work. I have changed the glow plugs twice since. I don't see them as related to the head gasket.

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'97 E 300 D