Valve stem seals get old and hard and fail to seal, usually last 100,000 miles or a little more. Can result in using a quart of oil in 1000 miles easily. Replacement doesn't require removal of the head, so I would do them before jumping into anything else. If you have compression leakage past the valves on a leakdown test, then valves need work, but not otherwise.

Be aware that on the earlier aluminum blocks, the threads may pull out in the block when you take the heads off, and it requires a rather expensive jig to install TimeSerts or Helicoils prior to re-assembly.

Meaning, of course, that if you don't have to remove the heads, leave them on!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!