A used/refurbished H2 body(or if I were you, buy a pair of them) and a Phase One P40+, to me, seems like a nice combo for headshots and portait work. "Smaller" sized chip, yes, but still a great sensor, and you still get to tether to C1. Not the newest tech on the block, but very stable, and proven platform(the + series backs that is). There is also the P30+, which is also a great sensor. They're also cheap as chips these days it seems. A friend just picked one(P30+) up with an H2 kit(80mm lens, prism, etc.) for $4000. It had ~65k actuations, so not "new", but still has tons of life left. He got 4 back batteries and 2 grips and 2 chargers for each in the deal. Talk about a steal, IMO...
H-series lenses(all leaf shutter) are available at pretty much every MF rental house/company, and are great lenses for the ~40mp sensor in the P40+, and equally good on the P30+.
IF you want "new" tech, then I'd say go for the 645Z. IMO, best bang for the buck going right now(if you can give up LS lenses across the entire range). Yes, you also don't get C1 file support, but that 50mp sensor kicks out some tremendous files, and at high-iso too!
-Dan