time to upgrade - advice sought

[Paul Krzyzanowski <paul@research.att.com>]

I've decided that it's time for me to shelve the Leica IIIf(rd) that I've
been using for almost a decade and upgrade to the M series.  My motivation
is: (a) I'm fed up with trimming film and loading it, (b) using an IMARECT
finder all the time is bulky, and (c) a bayonet lens mount is more convenient.

I'm thinking of getting an M6 or possibly something in the M4 family and
have a few questions. I'd greatly appreciate any comments or advice:

- are thread to bayonet adapters available? If so, does it make sense to
  get any for my current lenses? (I've got 5; it seems a pity to put them
  all out of service.)
- is there a difference in film loading between the M4 leicas and the M6?
- what are the differences between the M4, M4-2, and M4P? Should I want one
  over any others?
- If I get an M6, it seems that  I'll be spending on the order of $700 more
  (on the used marked) to get a camera with a built-in meter. It's handy for
  the times I'm too lazy to carry a spot meter, but is it accurate and easy to
  use? Do you find it indispensable?

Thanks,

Paul