Thanks, but where does the 0.72 viewfinder fit in with my lens choices?
Where does the 0.85 viewfinder fit in?
Yes, an 0.72 finder is probably the one for you. It provides frame lines from 28mm through 135mm. I used to use a 24mm on a 0.72 M without an aux. finder by using the very outer edge of the entire viewfinder and mentally adding a smidge. I hated using the aux finder, so 24mm was as wide as I went with M lenses ... but the 21 is a great lens, and using it with a finder may be worth it for you.
0.85 mag finder provides frame lines for 35mm on up. You would need an aux. finder that slides into the hot shoe for framing a 28mm, 24, 21, or now the new 18mm. Again, I used to use the fudging method to use a 28mm on my 0.85 ... it's just something I got used to doing to avoid the aux. finder.
There is also an 0.58 mag finder that eyeglass wearers favor which shows framing for 24mm lenses ... but it's a bear to focus IMO. However, you can add an additional eye-piece magnifier when using some of the faster longer lenses like the 50, 75, and 90mm lenses.
I prefered using 2 Ms: an 0.72 for 35mm on down and an 0.85 for 35mm on up.
BTW, for the ultimate contemporary M track down the MP3 ... the looks of the M3 but with a meter, etc. That was my favorite Leica M film camera of them all.