Ricoh GXR.
The GXR with Mount A12 permanently installed on it is no longer a modular camera but is a purpose built M lens camera, the only purpose built M lens camera on the planet that is in current production not designed and built by Leica.
The Mount A12 has no anti-alias filter, like your M9. Your lenses are free, within the confines of a cropped sensor, to express themselves.
Unlike the M8, the Mount A12 has no issues with IR. You won't have to get out IR filters again.
The GXR / Mount A12 is a 12.3MP sensor - not the most up to date but not that dated and it remains very serviceable. It's biggest weakness is in noise as sensitivity goes up, but that might be familiar to M9 shooters too so perhaps not an issue.
It may be that Ricoh updates the sensor to 16MP or beyond. There is a desire within the GXR community (and beyond, no doubt) to see Ricoh-Pentax come out with a full frame module.
The GXR plus Mount plus an EVF (which I'd assume you'd want whether you use it all the time or not) will run about $1,200 US and have a warranty. It won't be a rangefinder camera though.
For me the GXR is both a good camera and a way to keep using my rangefinder lenses until an affordable full frame digital option comes up.
I used the NEX-5N - despite having a 12.3MP sensor to NEX-5N's 16MP sensor, the GXR produces visibly more detailed files and better edge to edge performance, particularly with distant subjects. Close in, both cameras do excellent. Depending on what you shoot go with one or the other. A NEX-5N with the "Hawk Helicoid Adapter" allows you to turn your NEX/M lenses into a macro photography machine on the fly, very handy. I have a macro ring for the GXR (Leica ring) - it works but it isn't nearly as handy as the helicoid adapter.
Depending on which lenses you have you may wish to avoid the NEX-7. Symmetrical wide angle lenses don't do as well on the NEX-7 as they do on the NEX-5N; it may be the NEX-6 is different though - no results to talk about as yet but more should be known soon as Sony lets these out into the wild for review.
I do still want to get to a full frame sensor for my M lenses, but not at M9 prices new or used, and it's really the M that I would want anyway, being a fan of live view /and/ rangefinders. Probably I won't ever get to satisfy both itches with a digital camera. I'm ok with that.
The GXR isn't perfect, the NEX isn't perfect. Heck, even a M9 has issues. You'll need to think about what sort of photography you intend to do most with M lenses on a digital camera that isn't a rangefinder - through that thought process a choice should become more clear.
I ditched the NEX after a couple months and have continued to shoot the GXR with M lenses only. After a year, I'm still happy, but do want to see Ricoh bring out a full frame module. Here's to hoping.