If you are looking for something that will indicate when you are at the "correct" rpm to shift gear then it is often called a "shift light". I doubt there are any designed to be fitted to MBs but there are probably universal kits for this, getting the RPM information is fairly easy as it's available through the 12v ignition signal IIRC! The units usually require some sort of calibration and you have to set the RPM thresholds for the various lights yourself (not all engines are the same, obviously!).
Personally I'd not bother fitting one, for one thing the placement on the dash might not be so easy and it would never look classy but secondly right when the correct RPM to change gear is reached my car magically changes gear for me, seems excessive to have a lamp on the dash to tell me really.