Given that you asked such a question, I hope you know that it is manual focus regardless of which M series model you are using. Manual focus means that you have to make all the decision and that translates to how skillful you are. If you are good, I am sure any M Series will suit you and you will do greate justice to the hardware you hold in your hand.
For me, I can never shoot a street scene up close. As such, none of the M series works for me. I suck big time when it comes to street photography.
Some of the older Leicas have very small viewfinder windows. I think you are better off asking this in the RF section.
BTW, what is the attracting feature of using a RF over another system? (ie. SLR, DSLR, MILC, serious compacts)
With rangefinders, you need to be comfortable with the focusing system as well as with manual focusing.
Many folks use pre-focusing and zone focus to get over the need to MF fast or eliminate the need of focusing altogether.