The M104 engine does have tendency to eventually need headgasket replacement. Not sure why the cylinder head would need replacement with that few miles unless it experienced some sort of serious failure. The transmission fluid should be replaced at the regular intervals as indicated in your service book. You can drain a good portion of the fluid if your transmission still has a drain on the torque converter. I know Mercedes changed the guidelines in the late 1990s to a "lifetime fluid" but they eventually went back to fixed intervals.

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1989 300CE