The M5 was a sales dud when it came out, but recent demand for used ones indicates that it's become a collector's item and probably not a good bargain. The M6 has two versions -- the original M6 and the M6TTL. Both have through the lens metering, but the latter has extra contacts for controlling flash -- not a really significant improvement. Either one is reasonably plentiful on the used market. The M6 and MP have small classic shutter dials. The M6TTL and M7 have larger shutter dials that turn the opposite direction, which makes it more intuitive with the LED meter display. The MP has the original rewind knob, which is sturdier and less likely to break than the fold out crank on the others. The crank system is easier but really quite sturdy, so the MP advantage is not much unless you really shoot a lot or subject the camera to rough use. If you handle the MP beside the others, you will notice that the operation seems smoother and the controls have a bit more "finesse." Again, this is in comparison with the other fine handling models. Also, the MP is just a bit shorter. The M7 has an electronic shutter, and if your batteries run down, you have only 1/60 and 1/125 shutter speeds. The others have mechanical shutters and if you lose the battery, you lose only the metering.
I believe you're talking about quick focus and shutter adjustment for street shooting, so I would count the M7's auto exposure as a definite advantage. You probably don't need extra wide apertures, either, since you will be into fast but approximate focusing -- sometimes using zone or hyperfocal settings for speed. This implies that you can get by with f2.8 lenses since you will probably do most of your shooting in the f4-f8 range. Where I'm going with this is the lens choice. My favorite for street shooting is my f2.8 35mm Summaron. It's a compact and nice handling lens that produces excellent images. And 35mm is a very natural focal length for Leica photography. You might want to start with it and add others as you expand your system.