Hey congratulations on the leap to 645z
It's an amazing machine and wish you the best for creating amazing images from it
Talking about lenses, I started with a 55/2.8 new one with the camera (got a good deal on it).
Then I went for the 6x7 adapter and bought a used 105/2.4 lens. I now use this one for most portrait sessions when the subject has patience for manual focus
Most recently I got a fotodiox adapter for Hasselblad V to 645z along with a 150/2.8. I was trying to go for a lot of bokeh for wedding shoots however I have found the 150 to be very difficult to accurately focus wide open. My copy has a strange haze at f/4 only. Other aperture values are very clear and sharp. So the experience was mixed. On a tripod with enough time to focus, results wide open are amazing.