I'm not sure what type of images you are going to shoot but available light often means outdoor/street photography. The Rolleiflex 6008 is way to large, noisy, obtrusive and heavy for that type of photography - it works best for planned location or studio work. The Rolleiflex TLR is something inbetween those two. I don't own one so i can't really comment on it except that it's a lot smaller and is known to be quiet.

The best type of cameras for available light are the rangefinder cameras as they will always have a very bright viewfinder and you're able to compose and focus easily even when it's very dark. If you really need medium format, maybe a Fuji MF rangefinder would be a better choice? If MF is not a requirement, I would have chosen a Leica M.