Rob-F
Likes Leicas
i think you need to answer a few questions to narrow the field of choices. something like this:
need meter and auto-exposure? M7 (over budget)
need meter and ttl flash? M6 ttl
need meter? M6
no meter but need 28mm framelines? M4-P
no meter but need 35mm framelines? M2, M4, M4-2
no meter don't need 35mm framelines? M3
i'm ignoring non-leica body alternatives, though maybe you shouldn't.
That's a really comprehensive decision tree!
I can point out a few other things. The M2, M3, M4, M5, and early M4-P (and M4-2) have framelines that are sized for the most accurate framing with subjects at a distance of one meter. At longer distances, the framelines will be a bit undersized--that's to say, you get more in the picture than you saw in the finder. I've seldom had a problem with that, as the difference is not extreme. But . . .
The later M4P, M6 (in all its variants), MP, and M7, have framelines sized for best accuracy at 0.7M. This makes them noticeably undersized for shooting at greater distances. The 50mm frameline is especially undersized: it covers the same ground as my 60mm lens on my R6. That's pretty bad. Some of us now use a 40mm lens with the 35mm framelines, on our M6, M7, or MP, so as to have at least one lens we can frame adequately (I didn't say accurately) at medium and longer distances. When I want to shoot my 35mm Summaron, Summicron or Summilux lenses, I tend to use my M2. For 50mm lenses, it's the M2 or M3. For other lenses, I'm more inclined to use the M6 or MP so I can use the meter. I can always take meter readings with the M6 when using the M2! In my experience, the M6/MP is fine with the 28, 75, and 90mm lenses.
My favorites are: M2 with 35mm lens, and for the 50mm lens; M6 for best value; for the meter; and with 28, 40, 75, and 90mm; MP for similar reasons, except it is not the "best value" for the money.