If you want to stick to something that will take M-mount glass, another vote for the Zeiss Ikon here. If you aren't wedded to the M-mount lenses, also consider a Contax G2. While the manual focus on it is not as direct a system as on the Leicas, it can be focused manually, or auto if you want. It also gives you aperture, shutter, and program AE as well as manual, and it also has auto parallax correction in the viewfinder. Plus, the lenses for it are some of the best ever made, and cost a fraction of what Leica M glass costs.