Also replace the fuel pressure damper at the fuel distributor. You should see a small can shaped vessel in the circuit with the Warm Up regulator.
Here is a pic:
Can you or anybody explain what this thing actually does? And why there's a vacuum line on it? I've been wondering for the last 2 years and others may wonder as well. 
^^What Daniel said. It's not a vacuum line. It's a leak line in the event of a ruptured diaphragm inside. While the leak line will prevent raw fuel from spraying all over the engine bay and exhaust preventing potential fires, the other issue is that the inside of the engine will fill with fuel. So if you ever notice your oil level rising it'll likely mean the damper has ruptured. Also, the oil will smell of petrol and be very thin in consistency.
The damper itself is there to absorb pulsations from the fuel pump so there is a constant pressure on the fuel distributor and injectors. If pulsations aren't alleviated then the engine will run rough, drive worse, and deliver poor fuel economy. Some of the earlier models (D-Jet cars and some early K-Jet) had a primary damper at the fuel pump, as well. On this one there is no diaphragm inside. It's only a metal housing shaped to lessen pulsations.
Regarding your valve lash adjustment, you'll need a valve cover seal kit, part number 110-010-08-30, and you'll need a special wrench, part number 110-589-01-01-00, to turn the adjusters on the pivoting assembly that the rocker arms pivot on. The wrench is still available new from Mercedes for around $60-$65. While doing this job, do take note of the rocker arm surfaces where the camshaft strikes them. The rocker arms are notorious for becoming score due to the top metal layer being too soft (a factory mistake). I make note of this because if this issue is left to it's own devices the scored rocker arms will damage your camshaft lobes.....and then you have to buy new rocker arm AND camshafts. Rocker arms from Mercedes, part number 110-055-09-01, are $206 each as of right now, and if you had to buy all of them you'd have $2,500 just in rocker arms. By all means seek out aftermarket if any of them need replaced. Thankfully, any aftermarket rocker arms made for an M-110 engine are old stock and are made well. Anything new or recently made be very suspect of.
I make mention of these part numbers so you can research them on Google or take them to your local dealership and have them order the part numbers. Most dealerships will have no knowledge of the older models or that they can even still get parts for them. Heaven forbid you ask for a special tool.....they'll instantly tell you that it's no longer available. So use the aforementioned part numbers to your advantage and outsmart the others.
One other thing I want to touch on is sparking plugs. Be certain that you have non resistor type sparking plugs. You can still buy the correct, non resistor, sparking plugs from Mercedes for your car. Every place else will sell you the wrong plug. There are two choices you can use for your car, a Bosch W 9 DCO or a W 8 DCO. The W9 will be a hotter running plug, and is meant for normal driving because it has high heat dissipating characteristics. Short jaunts to and from destinations in which the engine might not get fully up to operating temperatures would require the W8 plug since it won't dissipate as much heat. The W8 part number is 003-159-11-03, and the W9 plug is part number 003-159-10-03. In essence, if when reading the nomenclature on the side of the sparking plug you see an "R" in the description then it's the wrong plug. The "R" stands for resistor. Your high tension leads already have a resistor in the metal housing that goes over the sparking plug. The ignition system does not need another resistor to overcome. It causes rough idle, lessened fuel economy, and puts a strain on the coil and switchgear box (Ignition control module) long term.
Regarding your check valve, which one are you referring to? There are a handful of check valves on your car all in different locations.