Johan is spot on for the M3 DS gears.
From all the Leicas, I or IIIc are the easiest to maintain and made simple but elegant (especially true for Leica I)...
As said by Antoine de Saint Exupéry
..perfection is attained not when there is nothing more to add, but when there is nothing more to remove.
The M's are easier to put out of alignment in most regards and pricey to repair.
The good thing is that parts made for the newer models like M6, MP could be used in the old M's but still a RF replacement by Leitz in Solms might go for close to 500EUR.