Hey Fred,
I can only comment from personal, film-shooter only experience, but I'll try to tackle some of your points here:
my questions to you(trying to clear my fuzzy brain up)
1. When you say "IQ(image quality)", what are you meaning exactly? I've found that for my work style, color negative films work very well for what I'm trying to achieve; the "look" I guess you could say. I personally like the Portra line of films from Kodak.
2. "Texture" (your Q #2), when you say texture, are you referring to film grain as opposed to digital noise?
Also, how large are you looking to print, and how many of those prints will you me printing/have printed for you?
3. From my experience with the Pentax 645 system, I like it. The only thing that irks me is that you don't have film backs, so you can't say carry a back with b/w in it, and another back with color. You'd have to carry 2 separate camera bodies, which can get bulky.
other than that, the lenses(mostly the most current Pentax designs) are great! They aren't as sharp as say hasselblad v or h-series glass, but if you're worried about longevity, and system stability over the longterm, I'd say look at hasselblad V(500 series bodies, 501 and 503cw). the lenses are super sharp(cfe,cfi and cf glass), and all the digi back manufacturers have the possibility of mounts for the V-system. lenses are super plentiful on the used market, and if you want to shoot film(lots of film+dev costs to cover the cost of a digi back, even one that's used).
I figure that over the last 2 years since I've gotten serious about my work(I'm 21 btw), I've probably shot about $2-3k worth of film(b/w, c-41 and e6) in 120/220 and 4x5(with some 8x10 occasionally). I develop ALL my b/w myself at home(easy if you haven't done it before), and c-41/e6 I have a line for cheap dip-n-dunk processing.
so, if you develop yourself(b/w, and even color), your costs can be very low.
shoot film, its great, you have something TANGIBLE that you can hold(if you're like me, I like things I can hold), and you can scan it at whatever res. you want!
Hasselblad V would be my best recommendation if you're looking to shoot film as well. Great system, no adapter plates to worry about(like with the RZ/RB's from Mamiya)
I shoot with an RZ67 btw, you can put a back on them, but you need an adapter plate.
-Dan