ques. about buying a used M6

[batista@gandalf.rutgers.edu (David Batista)]

        Hello all.  I am a Leica novice and I have decided to begin hunting
for an M6 and one of the 35mm or 28 mm wide angle lenses to go with it.  I
have poured over the ads in Shutterbug for 7 months, followed this list's
postings, and studied several of the Leica books (M Compendium, etc.) and I
have developed (no pun intended  ;-)  ) several questions.  
        Thanks in advance for your time and attention !

        1.  Are there any parts / aspects of the M6 that are prone to wear
or failure that I should be especially careful to look for when buying a
used example?  Were there any years or serial number runs that were more
prone to break down than others?

        2. Since I want to "use" the camera rather than just "collect" it,
should I go for what I sometimes see listed as a "user".  Or is "user"
usually a euphemism for  "broken down piece of junk" ?  Or, should I spend
the extra money for one of the much more common grades of  "exec" or
"exec+" ?

        3.  Do camera dealers (especially Leica dealers) usually have more
Leicas than they advertise in Shutterbug ?  That is, do they just put their
best stuff in the ads?   Is it worth calling around to dealers if I don't
see what I want in the ads?

       4.  I see a wide range of prices (e.g. $900 to $2,100) for M6's that
are all listed / graded as "exec".  Is this because of cosmetic flaws or
does term "exec" have widely different interpretations among dealers?

        5.  Do you believe the jump in price for a f2.8 over a f3.5, or an
f2 over an f2.8 is worth it.  That is, have you found the faster lenses to
be more useful and so justified in price?  Should I try for the fastest
lens I can afford ( like with computers where you should always try to buy
the biggest hard drive or most RAM you can afford)?
        Thanks again for your help !
        David Batista
        Rutgers University Law Library
        batista@gandalf.rutgers.edu