I'd love to try the Zeiss Sonnar on my p645. In any event, you might want to try a p67 lens with adapter as well (although possibly more expensive than a p645 lens)..
The 67 lenses will be slightly longer on the 645 and there were some good portrait lengths in the 67 lineup. Also a soft-focus lens!
Pentax 645 lenses are very reasonably priced on the used market, atleast I think they are. Compared to Hasselblad lenses that aren't 80mm planars