The rattle or clatter, possibly is the chain tensioner (has a timing chain, not a belt), relatively easy to replace, I don't believe anything else needs to be removed, it screws into the side of the timing chain housing on the right side of the motor. Has a pretty strong emergency-running spring in it though, so read up first.
The trans sounds normal to me, MBs usually have a pretty firm shift, makes the trans last longer (reduced slippage). But will shift even firmer when floored. If it is really severe the vac modulator is adjustable, but don't try to make it shift "like butter". Would really need to drive to verify if it's normal or not, it shouldn't be so hard that it'll chirp tires or anything like that. But will be quite noticeably harder than a typical American or Asian car, especially back when this 190 was built.
Yes oil pressure unit is called Bar:
http://en.wikipedia.org/wiki/Bar_(unit)
The valves ARE adjustable to a baseline setting, but not a maintenance item. They have a hydraulic compensator, which on an OHC engine like this, it's like saying it has hydraulic "lifters". They only time you get involved is say if the cam is replaced, or a valve job is done, or a compensator goes bad. There is no screws and studs or anything you can mess with, it's all inside the arm and need a special tool (holding fixture) and dial indicator and shim set to do it.
One thing to watch for, is this has a pair of catalytic converters in the engine bay, right where the pipes come off the manifold, those will plug up and can basically destroy the engine. Are easy enough to unbolt so you can look down in the pipes for signs of plugging. Use you best discretion on repairs if they are plugging (WHAT state are you in?
)
Watch the perches that the front springs sit in (the upper part on the body, inside the wheel wells) for rust damage, have seen these break off. Even though it's clean of rust, I'd keep an eye on them. Watch for rusted/broken springs as well, down low where the sit on the control arms.
I think there could eventually be a market for clean unmolested 190's, they are hitting the trash heap pretty fast and no one is noticing "just another 190" attitude.
Gilly