Above advice sounds good.
Are you sure you are seeing white smoke? At freeway speeds it can be difficult to distinguish colors of smoke from your rearview mirror. Any other symptoms? Is the mixture setting correct? Any pinging?
Don't condemn your valve guides until re-sealing them no longer works. I'm at 263,000 miles on my '86 300E and a fresh set of seals last year took my oil consumption from about a quart every 700 miles up to 1 every 2,500. New guides eventually, but not today.
Of course if the head needs to come off for a new head gasket, send it off for new guides and whatever else. Even if the head gasket seeps a little oil and coolant, does it need to be replaced if no cross contamination of fluids is present?
'86 300E (It loves the highway. Loves it!)