Only thing with the alternator-driven tachs is, IIRC, you have to calibrate it...which means that you have to know what RPM the engine's actually turning at somehow. Setting it to what the idle RPM should be is not necessarily good enough; what if your idle RPM's off one way or the other?

IMHO retrofitting a factory tach would look best on one of these cars, but an alternative is the TinyTach. Not sure where it could mount on an M-B dash without looking tacky, though. If I follow through with the Toyota conversion, I'll probably get one of these. Only $65