You have adjusted the valve clearances in your long period of ownership? Mercedes indicates they should be at least checked every 15 thousand miles. In extreme circumstances it could disable the third cylinder at hot idle. This is a regular periodic service item.

Actually it sounds like you have a lot of getting your hands dirty work ahead one way or another. If the car has excessive miles overall on it you might consider getting good a used motor and and changing it out. You may need a hand getting a good used one though with your experience level. The motor has to be removed anyways for the rear seal.

You have a lot bigger issues than valve seals currently so I would not do them. The two hundred dollar leak down test has a nice ring to it. Although if it is not a valve clearance issue. A little air injected into the third cylinder may indicate where the excessive leakage is for less money doing it yourself.