Should I buy a new M or older M3..6?
It basically depends on if you need the latest built-in light meter. If you don't then older Ms may suit you fine. Indeed there are those who believe the older, brass-body Ms are superior to the newer zinc-alloy-topped Ms. Cameras built with serial #'s greater than 1 million up to the end of the initial M4 run in 1974 are particularly well-regarded for their precision build and durability (hence the rip-off asking prices for 25 year old non-collector M4s).
Erwin Puts has put together an detailed www page which describes all the different M models at:
The following are a pair of links to introductory articles by Tom R Halfhill, published in Shutterbug Magazine in 1989, dealing with what to look for when buying 2nd hand M bodies and lenses. Mr Halfhill lays particular emphasis on how to save money by avoiding the kinds of things that collectors drool over:
Don't expect any massive discounts for older M series cameras though. Depending upon condition, maybe 50% off the price of a brand new M. Welcome to the Land o' Leica…
A word of warning about the M3's and M2's… these have removable film take-up spools into which you must thread the film prior to loading the camera. This can get on your nerves, and spare spools are now relatively hard to find. M4's and beyond have replaced this system with a much easier to use
rapid load mechanism. Just drop the film into the camera, close up, wind on a few frames and you're done.