Bodies: The M-P 240 baseplate is 2mm larger in the fore-aft direction than the M10-M/-R baseplate. Some of the protuberances on the M 240 body stick out farther than on the M10, that accounts the reported "8mm" thicker difference. The M240 and M10 differ in where you hold them by about 2mm in thickness. It's noticeable, but it's really not that much.
I've had M9, M-P 240, M-D 262, and now M10-R and M10-M. All feel pretty similar to me, in my hands and at my eye. The M9, bought new, went back for a sensor cleaning which turned out to be corrosion on the sensor ... I traded it up for the M-P 240. I bought the M-D 262 while I had the M-P 240 and just liked it more, so sold the M-P 240. I stopped using the M-D 262 after a time when I found the CL suited me better. Things change over time ... my eyes changed slowly over that five years and I found the EVF difficult to use in sunlit conditions. So I bought the M10-M to get the RF and liked it so much I sold the CL and bought the M10-R for my color camera.
Which one to buy? Well, unless you handle your camera like a basketball, you shouldn't need a rangefinder collimate and calibrate more than once in the lifetime of a camera. Buy three batteries when you get the camera, mark them 1, 2, 3 and use them in rotation ... they'll last for longer than you care to use the camera. The 40 Mpixel sensors in the M10-M/-R are a bit different than the 240/262 and M10/24Mpixel generation sensors in DR and noise, but beyond that, unless you're buying two cameras, you just get used to whatever the controls are and feel it, and enjoy it.
Go handle a camera or two and go with what your fingers and your gut tells you. 🙂
Lenses ... I use good lenses from Leica and Voigtländer that are properly rangefinder coupled for most of my shooting. Obviously, the Voigtländer lenses are far less expensive, and they're very good on imaging too. I had the Color-Skopar 28mm f/3.5 which worked beautifully on the CL and on my M4-2 film body, but it doesn't work all that well on the M10s so I bought a Summicron-M 28/2 ASPH at a reasonable price. The Summarit-M 75/2.4 works well, my old M-Rokkor 90/4 (same lens as the Elmar-C 90/4) works well too. Both reasonably inexpensive, although the latest Voigtländer 90/2.8 seems really good too, and is cheaper. Most 50mm lenses are going to be pretty darn good, picking which to suit you is more a matter of which taste you prefer. I have both Summicron-M 50/2 (current series) and Color-Skopar 50/2.5 (LTM with adapter) and both work beautifully; the Color-Skopar new was less than half the cost of the used Summicron. You pays your money and takes your choices..
I also have a few Leica R lenses (bought pretty cheaply when Leica discontinued the R system) to do long tele, table top, and macro work, and the Visoflex 020 to work with them. The M10 is better at being a pseudo "mirrorless" camera than the M240, but it depends how much you end up using such adaptations. I have also adapted Nikon F mount lenses (28/2, 55 Micro, Sigma 600/8 mirror) with good results. But I'd not depend on adapted lenses with an M, it's just too clumsy for a daily shooter using adapted lenses. Better to stick with M-mount lenses unless you're doing a lot of edge cases.
G