Having owned the SL and loved it, using SL, R, and M lenses on it, and sold it all for lack of use ... if I were to buy another SL series camera I'd go with the SL2 body first and use whatever lenses M or R lenses I had on it, then buy whichever SL series lenses I wanted afterwards.
I don't care what anyone else says about what works best regarding M or R lenses. I've used the camera extensively with both. Both series lenses perform extremely well, the difference is in ergonomics primarily. The R lenses' ergonomics suit the body's layout a bit better, and M lenses constrain somewhat the use of the camera's excellent TTL focusing/viewing capabilities due to their relatively limited close focus abilities, but both of these are minor issues.
Using the SL/SL2 with M or R lenses does not support AF, or Program mode, or Shutter priority mode. Again, IMO, it's easier
to focus with the SL than it is with the M—the viewfinder is that good, and you have superb focusing aids*(both peaking and magnification) that you don't have with the M. So, for me, the lack of AF is far less significant than many seem to imply: I have (always had) crappy eyes and I can prove that my focusing with the SL always outperforms my focusing ability with the M cameras by pointing to several thousands of exposures with both.
Regards weight and size, the SL is a larger, heavier camera. It's 10mm wider, and 30mm taller to the top of the EVF hump, compared to the M. The body measured in absolutes is deeper too, due to the protusion of the viewfinder ocular and the built-in grip, but where you grip it is actually about the same depth other than the grip itself. The largest difference is that it's about a half to 3/4 pound heavier, kind of the typical difference between any M and a professional grade SLR body (like a Nikon F4 or Canon F-1 whatever) historically. The additional of lens weight depends on what lens you use, of course, and many M lenses are both small physically and light in consequence. Summary: If you like how a pro-grade SLR or DSLR feels, the SL fits your hands the same way.
Should you spend the money for one?
This is the key question, of course, and only you can make answer to that. What are you looking for? Why is such a purchase of interest to you, personally?
I loved the SL and, when I bought it, its capabilities and advantages over any other digital camera I had used were an immediate win for my photography. As time went on, and I retired, and I closed down my still existing but small photo business, I found myself not using it other than for specific niche capabilities in macro, tabletop, and long lens work that the M is simply unsuited for. I decided that such an expensive collection of equipment as an SL body and a pair of superb SL zooms that I had was foolish to hold onto if I wasn't going to use them a lot and sold them off. I bought a CL body to use with my R lenses for those niche capabilities ... the smaller format and lighter/smaller body were actually a plus for them over using the SL in some ways. Over time, I've found that the CL does what I want for such a large portion of my photography such that I hardly used the M-D at all any more, and I sold the M-D262 as well.
There is a still extant bit of my photography that could benefit from a larger format camera and after much rumination and consideration, I decided that I needed a larger format to really address it the way I want to. And by larger, I mean larger than the difference between APS-C and FF. Since I already have a complete Hasselblad 500CM film camera kit, the sensible thing for me to do was to extend that with Hasselblad medium format digital equipment rather than buy back into FF format digital gear. It's a personal choice, both objective and emotional, but I simply prefer the Hasselblad "box with a lens on the end" form factor over the hand-held 35mm SLR or RF form factor for that work. I look forward to when the 907x Special Edition I ordered arrives.. And I've taken the 500CM out of its long storage and been shooting with it, have revisited how much I love it. It's the right thing for me to do.
You have to come to grips with what you want, and what's going to do your photography the most good. Sometimes, buying nothing is the biggest plus, putting your energy into seeing, traveling, and thinking of photographs rather than what equipment is best. And sometimes, getting that special camera that inspires you to see differently is the right thing.
"No matter where you go, there you are."
Hasselblad 500CM + Distagon 50mm f/4 T*