There is a somewhat similar thread running here right now.
To me, the first major break is: meter, or no? If yes, the leading choice is an M6. They are plentiful, are built on the classic M chassis, can be found with some frequency in "user" condition, and aren't subject to a variety of the foibles presented by the M5 (which I love) or the CL (which I also love). Used prices probably start around $800, though. The M5 is no cheaper, sometimes more expensive, due to collector pressure, and unless modified needs some kind of workaround to deal with the lack of availability of the mercury cells it was designed for (as does the CL). Not hard, not terribly expensive, but a factor perhaps. Neither the M5 nor the CL are built on the classic M chassis, either, and though their exposure meters are excellent they are not getting younger. Be cautious about early M6s (marked "Ernst Leitz Wetzlar GmbH" on the top plate) because unless repaired the metering circuit can fail, which is an expensive fix. The M6TTL and M7 are probably out of your price range.
Non-metered bodies. Sort first by intended lens usage. M3 has the best finder, if you don't want to use 35mm or 28mm lenses. The M2 covers 35, 50 and 90mm. Both load more slowly than the M4 and later bodies, and also rewind with a (slower) knob rather than the (more fragile but faster) fold-out crank. The M4 adds a 135mm frameline (tiny). The M4-P and later add a 28mm frameline (almost at the borders of the viewfinder) and a 75mm frameline (hard in my opinion to pick out from the 50mm lines but I think I'm a minority of 1 on that).
Sort by perceived build quality and place of manufacture. Germany is always good, Canada is always bad, except that Germany following Canada is not good either, unless it's an MP. Huh? Ehhm, probably nonsense? They're all great cameras. The number of electrons consumed in that debate is beyond counting. My recommendation: forget it as an issue. Buy by feature and price.
Remember that almost all of these are getting older. Even M6s can be more than 20 years old at this point. Budget a CLA. Buy with return privileges and, if from a dealer, with a warranty.
There are so many different opinions on your question, most of which are valid. I say: figure out what viewfinder you want, find one in your price range, load it up and get out and go shooting and forget the internet debate about nuances that are mostly in our minds.
For a summary of the models:
http://www.cameraquest.com/mguide.htm
For a "checklist" on buying used:
http://www.cameraquest.com/leicamchecklist.htm