I have an old 135/4 Elmar, which I use on the ZM either with the Leica 135mm finder or an ancient Zeiss turret finder. Either works OK. Focus has not been a problem, though given the fussiness of focus (especially wide open) I find I'm much less confident using an external finder with the long focal length... at least for moving subjects.
Using a Visoflex with the 135mm on the ZM is "fine" too but only if you're the kind of wingnut who likes the Visoflex... and are willing to noodle around with lens heads, adaptors, etc. just to have the world's least-convenient SLR. I do it and that is direct evidence you should consider any advice I might give as absolutely idiotic.