I bought an M4-P to use with my 28mm Ultron, it's a great camera although some don't like the cluttered framelines in the finder vs. the M2,3,4. Some also say it isn't as smooth as the M2,3,4 due to the use of brass gears in those models and steel in the M4-P, the change was made so the M4-P could use the winder without fear of stripping the brass gears. I can tell a difference between my M3 and M4-P, be it ever so slight, but it's more like splitting hairs. If you like to shoot fast lenses like the summilux 1.4 or faster, you might consider and M3 or M6,7, or MP with the .85 finder, they will be more accurate, though I have used my 50/1.4 with the M4-P and it worked fine.
You won't go wrong with any M body as long as it fits your needs (ie. framelines for the lenses you use most), good luck.
Todd