When I decided to get back into shooting with rangefinders, the first Leica I got was a clean, used M4-2 and a Canadian (Ver. 3) Summicron. The viewfinder window was fogged (fairly common), so I sent it off for a CLA. Although the person who repaired it said the M4-2 was the worst RF camera Leica ever made, it came back working fine and continued to do so for the next couple of years until I sold it for an M6.
Why? Because I wanted the built-in meter.
The M4-2 was the first Leica RF with the cheapened rangefinder that will flare out in bright and backlit situations. This fault was finally rectified with the MP. DAG will drop an MP finder in your M4-2, M4-P, or M6 for a little less than $200. It may actually be a tad cheaper if you have it done with a CLA, I don't know.
Considering the prices for M6 bodies these days, an M4-P, unless really cheap, doesn't make sense to me. An M4-P is essentially a meterless M6. If you're thinking M4-P 'cause you don't want/need a meter, get a good user M6 and take the battery out.
🙂
Otherwise, you're on the right track: get an M4-2, make sure it works properly, and go burn through a mountain of film. You'll love it.