I've never adapted an M lens, but I have adapted some R lenses for my Canon dSLRs, and I've also adapted some Olympus OM lenses on my micro four-thirds cameras. And I'd say it all depends on which mirrorless camera you end up getting and what type of manual focusing assist features it offers.

Right now, the latest generation of Fuji X cameras (except the entry-level X-A2) has three ways to assist you with manual focus: magnifying a portion of the image in a "patch", focusing peaking (where the areas of highest contrast in the image are highlighted with a color so you can more easily determine what's exactly in focus), and split-image overlay patch--similar to what you're used to with a Leica M-mount rangefinder. Fuji, as far as I know, is the only one to do this with a hybrid viewfinder and LCD overlay "patches". You may want to consider moving from the X100 to an X100S or X100T. The X-Pro1 only does magnification, as it's much like your X100. I would speculate, however, that if Fuji ever gets around to releasing an X-Pro2, it will likely inherit the focus assist features of the current generation of X cameras, but for now, an X-Pro1 might frustrate you.

Most other mirrorless cameras (e.g., Sony NEX) allow you to do enlargement and focus peaking via liveview of the LCD or the EVF. Split image/overlap is rarer. And the older the camera, the less liable it will be to do anything but magnification.

You can, of course, always simply zone focus, but since M lens DoF and focus scales are calibrated for film and full-frame, unless you're going for a Sony A7 full-frame model, it may take a little experimentation to figure out if adjustments are required.