Don't overlook picking up a Hexar AF for about the same money. Great lens and build quality, easy to use, and in some ways better than a Leica (or at least a nice compliment.) I am actually considering getting a second one.
I had a M6 and Summilux, which I do miss as they were lovely in the hand, but when I think about the kinds of photos I would use it for - quick, stealthy, low light - the Hexar is the more practical and probably more successful tool. As much as I like the idea of using a RF for focusing, it is slower than AF and sometimes more difficult to use.
The old Canon are great too, especially if you are lucky enough to find a good working model or get it CLA'd. The old mercury cells are still on eBay - I think buying the original batteries makes more sense than mucking around with Weins or adjusting the meter (not!?).