Here's a quick set of instructions - this problem with the PCV will affect loads of four-pot M111 Mercedes, I think the supercharged ones make it more obvious. Really not a bad job to do.
Remove the pipework between intercooler and throttle body, including the MAF, keep the MAF wire out of the way.
Remove the throttle body, the little mesh thing which is also the gasket for throttle body to metal insert on manifold and the metal insert - all hex bolts. Three on the inlet elbow which you'll probably do when you do the pipework, one underneath it which is shorter, then three good chunky ones, all allen key heads (6mm, I think. Maybe 5mm). The brake booster pipe is attached to the throttle body mount, that's 19mm, there's also a vacuum pipe on the throttle body to the evap solenoid (I think. Pretty sure that's what it is).
Take plug cover off and disconnect the LT wires to the coils.
Release fuel system pressure - take fuel cap off too - then clamp the fuel hose in the engine bay and disconnect from injector rail. You will need to disconnect the fuel return line as well, as it blocks one of the bolts.
Undo the two E12 or E10 (sorry, wasn't making notes) long bolts which hold the fuel rail.
Disconnect the Cam sensor wiring and unplug the vacuum hose from the injector rail.
Release the clips and move the wiring loom guide back slightly - you may need to disconnect the cam adjuster magnet connector, but I had enough slack.
Undo the 13mm bolts holding the manifold, get a bungee cord to support it.
There's a section of loom going to a sensor in the crankcase and the crank position sensor. I undid the wire going to the crankcase so it wasn't under tension.
Now you have access to the PCV plumbing.
PCV valve on the clutched supercharger setup - probably the same on the post-2001 layout too - has one large formed pipe that runs to a point near the A/C compressor and has a thin wire cable tied to it, and two small hoses that go into brass nozzles in the cylinder head. The nozzles and chambers they terminated in were absolutely pitch-black and gummed up. They're a press fit in the head, and they're also brass nozzles, not something that degrades so although I bought two spares I had no intention of fitting them unless I had to. Some gunk and a toothbrush cleaned up the area, then I poked through the jets with a straightened paperclip - it's a 1mm bore restriction so you could use a drill bit but it may snap.
The clip broke when removing the old valve - order a clip. It's the sort where you push a pin through to lock it into place.
Once reinstalled it all looks neat and tidy. You can re-cabletie the wire that runs down alongside the PCV.
Change the inlet manifold gaskets, clean the mating face, the usual things.
Reconnect the wires you disconnected that are under the manifold.
Reinstall manifold, tighten the bolts to 15lb/ft (20Nm) - mine had thread lock on and I was out of thread lock, so I'm hoping the old material and correct torque will be enough. Start in the centre and work out to ends. Watch the centre bolt doesn't trap the rigid vacuum line to the fuel rail.
Replace long bolts for fuel rail. Replace throttle body mounting - don't forget the brake booster rigid pipe and the bracket to support the wire channel. I think the bolt for this was longer than the other two, but can't remember.
Reconnect fuel lines and fuel rail vacuum line.
PUT THE MESH THING BACK IN. Guess who forgot the first time. I'd get a new one, I hadn't realised it was in there - the rubber seal didn't feel like it was compressing much and caked oil was on the edges.
Put the wiring guides back into place, including the coil leads, make sure the guide for the front coil is in the correct, low position rather than interfering with the plug cover.
Reinstall throttle body - I put the short bolt in, then loosely put the other three in whilst tightening the short one, then attach the elbow with new gasket.
Check the two vacuum pipes on the right under the manifold, and double check you reconnected the other end of the black one to fuel rail. I think you could do this without disconnecting if the centre bolt were not obscured by it!
Clean the MAF. To sort this properly you should remove the bumper and clean the intercooler, which will be contaminated with oil, but I wanted to get the car back on the road so am leaving that for another day. I think having sorted the source of the oil, what's in the intake will eventually get drawn through the system, but I'm not counting on it and will clean the inlet manifold when I change oil and filters, and at that point - when I'm satisfied the car's PCV system is working 100% - I'll fit the Bosch MAF I bought.
Replace intake pipework, look all over the engine bay for any tools you might have missed, spend 20 minutes getting really worried that the little screwdriver has vanished, then when you're SURE it's not in the engine bay, take a test drive.