Upload & Sell: Off
If you go with an M-E or used M9, you'll want all the lens speed you can get. With the M240 the file characteristics at higher ISOs are similar to what you're already accustomed to with Canon, but I find that while one can push M240 high ISO files moderately, they feel somewhat thin and brittle. It doesn't take too much for white speckles to appear at pixel level and shadow values will start to reveal a green tinge. Unlike the M-E/M9, it's not such a great idea to push M240 shadow values a huge amount, especially at higher ISOs.
Also not sure how much you rely on fps or rapid shooting with your Canon kit and whether any of this will be relevant to you in a full Leica transition, but the M240 drops to about 1fps and half the buffer depth at ISO 1250 or higher. Since my shooting style seems to be quite different from yours, all I can say is it annoys me tremendously and is probably the greatest flaw in the M240, for me. It's not that I always need to shoot many frames in sequences, rather, it's immediate next shot readiness that I value in high fps cameras.
Since 99% of the time I'm sticking to ISO 1000 and lower on the M240 in order to avoid the 1fps limit, I still find f/1.4 lenses to be invaluable.
Something to consider about Leica's 21 and 24 offerings is that you'll get speed and character from the Luxes, with optimization primarily for wide open shooting, whereas the slow(er) Super-Elmar and Elmar will give you optical perfection at all apertures. The latter two lenses (though I have much more experience with the 21 Super-Elmar than the 24 Elmar) are very easy to use, without any catches. You just focus and the image will be sharp. With the Luxes (again, my experience is much more with the 21), image quality will suffer somewhat if you want to place subject content off center while stopped down 2-3 stops.
I often shoot at around f/11, if only for the sake of knowing there will be more than enough depth of field, even if there is some slight trade off in diffraction softening, I've found some scenes truly will benefit from the additional depth of field at f/16 (thinking specifically of the 28 Cron and 21 SEM), especially when aiming to include near and far scene elements. If the foreground content is relatively prominent, then that is where I will bias my focus. If foreground content needs to be tack sharp, I will either focus on it, or slightly behind so that it remains well within depth of field, and will let the background go a touch soft in exchange. If it's a good composition, then it will be worthwhile focus bracketing a few shots to ensure you achieve the expected results.
A while back Roger Cicala of Lensrentals made an interesting point when discussing lens test results. Often diffraction degradation is seen as something to be avoided at all costs, meanwhile many photographers don't hesitate to shoot wide open. In his lens test analyses, it was very clear that (across-frame) lens performance/resolution is generally better at f/16 than at near wide open. While some image degradation will be visible due to diffraction, careful sharpening in post can usually compensate.
The greatest annoyance of f/11-16, IMO, is dust. For whatever reason, I've been having a heck of a time with dust on the M240's sensor. Not just spots, but larger fibers on a relatively frequent basis...
Many of my lenses have a slight back focus bias on both my M9 and M240.. while you won't like my suggestion, I find on-the-fly compensation while shooting tends to work pretty well.