Hey, congratulations on the M6! It'll be with you as long as you want, I think, and will always be worth the occasional tune-up. (I have an R2A as well, and it is wonderful, but frankly I do not expect it to last for 30 years!)
The only 35mm I have used with either camera is the current Summicron ASPH. However, I notice that on ebay right now, there's a Canadian Summicron being sold by an LHSA member, and it has been recently CLAed by Leica. That would be a great buy if you got a good deal on it.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=30063&item=7511609421&rd=1
General comment, if you don't mind: I would get something on the fast side considering what you said it's for. At least f/2. Remember that available light means dark shadows, and you want to be able to get enough light in there from the shadow areas to get over the film's minimum sensitivity. That's my preference anyway. Otherwise the shadows will be very dark and the contrast will be accentuated. Some photographers associate this look with available light shooting, but I think available light photos are more interesting and have more depth with more even lighting. So in my case in those situations, I shoot wide open a lot, as wide as the lens will go. Although there are many good 35mm f/2.8s I don't think you would be happy with one in night shooting or other available light.
I have not used the Cosina Voigtlander lenses, but lots of people here like them, and they have some fast wide-angles. One of them is a 35mm f/1.2! It's a big mother, but that would not stop me if I could afford it. A 1.4 lens means that (with a Leica or other good rf and a steady hand) you can shoot at 1/8 or 1/15 in very dark situations and get away with it, instead of 1/4 or 1/8 at f/2 and not get away with it. A 1.2 might be enough faster than a 1.4 that in many situations you could shoot at 1/15 instead of 1/8.
A used 35mm Summilux would be nice, too, but it'd be so expensive I might go back & look at the Voigtlander store.
