1. Plugs on the picture look ok for 30k conventional plugs, excatly the condition of the plug to be replaced.
2. Iridium plugs more tolerant to the residue and better self clean then conventional (allowing for longer intervals)
3. M119 has tight clearences and is not that different from modern cars, even more, it has much less carbon build up then modern DI cars, especially german. They are falling behind these days with carbon build up big time.
Modern engines have different sump ventilation system and different programming, they bypass (blow by) more oil, which is being burned creating more deposits. It is done to reduce friction, reduce pollution and improve mpg. This is why oil standards grow like grass on the rainy day... etc. I don't want to write more about it, enough is written on-line. But new engines produce more crap then old ones in the form of the deposits. Exception is only lexus/toyota with their patented dual injection (di and usual the same time).
4. Based on the picures, i'd suggest to replace o2 sensor if it is old, if you have replaced it, may be clean intake and throttle body (amsoil power foam). Exception, if you have more then one sensor (cars after 1996?)... may be just test them, kind of expensive to replace.
5. Based of what i've seen on my car and many other non-mb cars, your plugs are somewhat more black then they need to be, but not much. Mine came out the same (do not know how nany miles they were in). I blamed o2 sensor and throttle body, this summer i can say for sure.
My idle issues confirm this guess, mixture and plugs are interconnected: worse spark changes burning and mixture, wrong mixture darkens plugs, more unburned fuel deposits on the o2 sensor and so on... if you tell us which components have not been replaced for a while, we could guess better