I currently own and use 6000 series cameras and have used a 2.8 Xenotar C series TLR. I was unable to find a replacment viewfinder screen for the TLR that was anywhere near as bright and useable as the 6000 screens with an 80/2.8 attached. YMMV.
I'm not going to touch the 6008 vs TLR issue BUT I can say I'm a big Rollei TLR fan with a couple of F's with 3,5 Planar, a 2,8 Planar, and a pair of Tele-Rollei's with the 135 f/4 Sonnar. Fabulous cameras as if you would expect any other response. Excellent with my favorite Ilford FP4+. Love the TLR's but the focus screens are kinda lacking EXCEPT one of my 3,5 Planars (12/24 model) has one of Bill Maxwells Split Image focus Screens--Nice and bright and contrasty and on par with my Hasselblad Acute Matte Split Image---EXCELLENT and I shuffle it to the Tele models when using them because it is that much better but then I do prefer the split image focus. As for Mirror Slap--I've never noticed any issues using my Hasselblads (501 and 503) and vibration BUT can't comment on the 6008 Rollei but I expect no issue---If not using strobes with the Hasselblad I do tend to trip the mirror in advance--kind of habit forming!! My suggestion is try a RolleiFlex TLR with either the Planar or Xenotar lens--I have a couple of Tessar cameras but I really like that very sharp and shallow DOF characteristic of these two lenses--not quite a Tessar trait is it!! If I could have only one MF camera and one Focal length then the 3,5 Planar Rollei TLR is my 1st choice but then my Hasselblad with 100mm Planar is mighty, mighty fine!!
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.