Can I ask why, specifically, you need a Leica? I had a Leica M2 with a 50 Summicron once, and even shot with several Canon Cannot GIII QL17 rangefinders with the 40/1.7 lens, and for me, I'm not sure what the attraction is to the rangefinder actually is. I thought it was me, and tried a Fuji X100s and that experience was WORSE.
If you have to have one, I agree that you should find a reputable dealer who is willing to guarantee their work and provide you with a working copy. Avoid anything that's been abused. Of course, this probably means you'll spend more money on it. But the bigger deal is that you have the lenses for it. Everybody here will say the body is just the light-tight box that holds your film, the LENS is where all the action takes place and enables you to record your vision (duh, right?). So I would think long and hard on whether or not the Leica M is what you need or not.
Heck, I think a better deal is to find some older Canon or Nikon stuff. The Canon FD stuff is really cheap used these days, and all of those lenses are great. The Canon F1, and the Nikon F2 or F3 are total workhorses. That brings up another thing about Leica, and someone correct me if I'm wrong, but with whatever body you have, you're kinda' limited to the different frame lines you have built-in, right? That's why pro Leica guys only have maybe 2 or 3 lenses anyway. I saw if you get the Leica 21mm lens, you have to get that viewfinder for the hotshoe. What a PITA, I say. With your standard 35mm SLR, you can use all lenses that manufacturer makes and have no problem using them. So I dunno, I appreciate a cool classic camera as much as anybody else, but in the last five years I've been away from APUG, I've been out doing shoots and solving problems having to do with lighting and all that, going for certain looks. The last thing I want is a camera that sorta limits what I can do or what I can rent for it.
EDIT - maybe I sound a little too bitter about it. I do like Leicas. Every time I see the work of Ralph Gibson, or Andy Summers, or Cartier-Bresson, Winograd....I'm always floored. But heck, I have that big Andy Summers coffeetable book he shot while on the road with the Police in the early days and that work was stunning. But I think he shot most of that with a Nikon FE, though.