Joe--
I'm not trying to talk down to you or suggest you are not sufficiently informed.
Cameraquest is both a sponsor of this website as well as a wealth of information. Consider looking at the
M Guide on Cameraquest for details specific to the camera(s) in which you are interested.
http://www.cameraquest.com/mguide.htm
The essential difference between the M4-2 and the M4-P, and I have owned and used both, is the finder. If you are planning to use but the 50mm lens, then you may find the coincidental 75mm marks in the M4-P mask set bothersome, thus the M4-2 might be a better choice.
Be mindful that during the production run of the M4-P the VF glass went from recessed to flush with the front of the camera. Many prefer the earlier VF as the flush fit finders exhibit a greater tendency to white out with flare.
If you want perhaps the single best VF for only the 35, 50 & 90 lenses and a removable reloading spool instead of the Tri-Y fixed spool of the M4 & later bodies, then the M2 could be a consideration.
When purchasing a used Leica M body it is worth the effort and perhaps added expense to buy with an understanding that it can be
returned for full refund within 7 - 10 days. As soon as you get your new to you camera run a roll of film though it shooting both in and out of doors. Test the rangefinder at near, mid and far distances, test the shutter at most if not all speeds, particularly 1/1000 & 1/500 as well as the slow speeds. Leica M bodies can loose shutter accuracy if not used for long periods of time, but they typically can be restored as well. But if you don't want to buy a project camera, test it as soon as possible and if need be, return it and keep looking if it isn't a keeper.
One thing to keep in mind, regardless of buying a brand or a specific camera, are details such as the effective RF base length. If you plan to use very fast lenses wide open, the effective base length of the Leica M cameras (except maybe the .58x finder) is longer than that of the Bessa cameras, providing an edge in focussing accuracy.
--JSU