Thanks, all - I've now made up my mind!
A Bronica Zenzanon-S 150/3.5 is on its way to me, and I'll build up the rest of the kit as money comes in. I may get the cheaper SQ-A and live without bulb and 16s.
I liked what I could see of how this lens renders a scene, and I wanted to get away from 'duplicating' what my digicam does, i.e., 4:3 format with a slightly-wider-than-normal lens. So I reluctant to get 645 and an 80mm, despite liking the other cameras. And I like the idea of using a WLF to keep down weight and bulk, so 6x6 makes verticals easy and I can shoot square or rectangle as needed. And when the money comes in, a 40mm will be nice, too...