A) Used normal (not special edition/material) Leicas are expensive relative to your other cameras, but they are not "expensive". A body is as much as an entry level DSLR...600-800 dollars.
B) Get a Voigtlander lens if you can't find/afford used Leitz lenses...a 35/2.5 or 35/1.4 for an M2 is a sweet little package.
C) Have you used a Leica before? They are, after all, just cameras. Built to an exponentially higher standard than something like a Canonet, but still just a camera.
D) Best finder magnification, in my opinion, is the .72ish range. Allows for 35mm lenses, feels good with a 50, and I don't use anything longer than that so couldn't care less about what the 90 frame is like.