Hi Wilko,
The 60-120 mm may be a relatively rare lens, i.e. perhaps not many sold. But it is by no means a lens difficult to get. It is still very much part of the Hasselblad lens line, still available brand new. ;-)
Yes, there are 3rd party teleconverters available. But at present there are three "official" Hasselblad (non-Zeiss) teleconverters in the Hasselblad program too.
Who is building these things is something Hasselblad is keeping secret: i asked, and asked again, but they just will not say.
But i'm pretty sure it isn't Zeiss. Given that on more than one occassion Fuji appeared to be Hasselblad's Japanese "partner in crime"...
I'm quite sure too that Hasselblad's mr. Per Nordlund was involved in designing the things.
I tried a Vivtar converter a very long time ago. Though resolution was still good, contrast too still acceptable, there was quite a bit of distortion. I never tried any other teleconverters. Just didn't need any.
The ArcBody was an attempt to bring movements within the reach of the Hasselblad photographer. The attempt before the ArcBody, the FlexBody, used Zeiss/Hasselblad lenses: good enough for tilt. no good at all for shift. Image circles too small indeed.
So they invented a miniature view camera, the ArcBody, and offered Rodenstock lenses in focussing mount. Whether or not they were "off the shelf" lenses, or speciall designed/adapted designs i don't know.
What i do know is that this thing was very poorly integrated, not part of the Hasselblad system: it only took Hasselblad film backs. The rest (camera and lenses) you had to buy all new.
Given that most photographers needing movements already own one or several "all moving" large format cameras, lenses, and roll film backs, or could get such a camera with the necessary lenses and film backs for far less than they were asking for an ArcBody... exit ArcBody.