All of them are great cameras, as we all know here!
Having said that, I find the M4 to be the best ("?" - Maybe I should say most pleasing...) ever made Leica!
The company was still in full strength (meaning the rangefinder was still going strong) and they could refine on the experience of the former models. It's a wonderfull match with a 35mm (you can't say that about the M3...), and still usable with the 135mm without googles (the finder markings are negligible, don't disturb when you use other focal lengths).
The finder is great! The frame lines are, in my opinion, much better in the M4, and M2, than in the M3, meaning that I don't much like the round corners on the M3 on the 50mm frame (the M3 is obviously better on longer focal lengths!).
Look how beautiful and well made the engravings are: that shows how much care was put in that camera!
I find that it is much better to rewind the film than in the older models, and I find that the form of the advancing lever is much nicer, somehow faster to go to the next shot: I also don't mind at all the black plastic tip. Actually find it much more confortable.
The camera will be some years younger, beeing nevertheless a true Leica classic!
This is my general opinion, but in your specific case maybe I would decide a little different, as you say that there are signs of wrong maintenance from a former owner... I think that only holding all three cameras in the hand I could give you a honest opinion.
Your specific case apart, my vote goes for the M4!
Cheers,
Rui
AL-MOST-LY PHOTOGRAPHY
P.S.: Very important for your decision, should also be which lens you use most, due to the difference in the frame lines.
I