Two parts to this:
- does the Rollei system make sense at all, given what else is out there?
- within that system, is 6008AF the right choice?
Have used Rolleis since 1992. The system has a good deal of depth, in lenses and other additions (macro, viewfinders, film backs, etc.). The beauty of the lenses is that there is a choice of Schneider or Zeiss (the Schneiders are a bit more modern), built like tanks, leaf shutters. Mostly MF, but there are some AF lenses - 50, 80, 180 and a rare 150. Use of the 1.4TX on the 80 makes a nice 110. There is also a good manual confirmation built in, so that use of MF lenses is pretty easy. And the lenses were top in their time, probably showing a bit of CA at the edges. I use the 50AF as a good copy lens, with its minor barrel distortion easy to dial out.
That said, there are other systems more current. You'd have to make sure to update the batteries, not too hard to do. I think DW replaced DHW, so there is some service still out there. Eric at RolleiflexUSA was big into these, check with him.
The idea of 6008AF as a film based camera, with AF capabilities, makes some sense, but know what you are into. The AF is OK, not 35mm standards, but quite usable. For slower portraits, fine. Not sure about higher speed. The camera isn't small, its not too heavy with film. I used to travel light with a 6003, thinner back, and flip up viewfinder, and found it OK, especially with the removable handle. The Hy6 is a bit more modest in size, but the handle is awkward for travel, and as you say, the 6x6 backs are hard to find (although a rumor was they were going back into production?).
As to any digital backs, the 6008AF is limited to the rare DP20, a fat pixel Phase One back. The Hy6 has a few more modern backs that will work, the Leaf AFI series (5,7,10,12), and some of the Credos - the 60 is quite nice. Not easy to find, but not impossible. THe earlier AFI series integrated nicely with the Hy6, the Credos a bit less, but still quite usable.
In short, its a system that will work well with film, good lenses, wide range of options, not too expensive these days. Negatives are limited knowledge out there, service is a bit thin (but still around), limited digital pathway, and the lenses are sizeable and a bit heavy.