Hi Darko,
I haven't shot with the ƒ/3,5 M Elmar, it might be identical to the ƒ/3,5 LTM Elmar made around the same time during the 50's.
You are having a second thoughts about the M ƒ/2,8 You wanted? You can always go Zeiss Sonnar ƒ/1,5 way