Hi!
The fixed lens range-finder style cameras generally offer very good performance
relative to cost, the P645 whilst a wonderful camera is IMO too much like the
kit that you already have without a really substantial advantage. The RF style
cameras optics are generally better than SLR optics, there is less need to
compromise also as you mentioned the leaf style shutters do not shock the
camera like a mirror/FP shutter (though the P645 is very good in this regard).
If I'm shooting at normal daylight shutter speeds with my Mamiya 7 I can't
detect a difference between images (at 40X magnification) shot on and off
tripod.
Bottom line is that really I wouldn't suggest that you bother with MF unless
you are considering 6x6 format or larger and modern optics that is unless you
aren't chasing improved image quality over what you already shoot.
I am chasing improved image quality of course... Otherwise why bother...
So, if I understand you correctly Fuji 645 that I mentioned is a
reasonable way to proceed, right?
Boris