Sorry for the delay in answering your question. I'm in Europe and find it very challenging to connect in Hotel rooms.
The experience I have with the valve seals involve records of the problem and the repair before I purchased my car. The records consisted of every receipt for every repair ever performed.
I do know that the oil usage went from a quart in hundreds of miles to virtually NO oil usage. I questioned this and found that there indeed was a SEVERE problem with the M103 engine.
It's hard to imagine a leaky gasket contributing that much to oil consumption. An oil leak so bad that it caused a quart of oil loss in a few hundred miles would probably absolutely paint the entire engine compartment and smoke while it was doing it.
The valve seals cost less than $10 from Mercedesshop parts shop and are replaced with a few hours labor. I would consider doing this before more major repairs unless the engine compartment has a nice solid coat of engine oil.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in