the linhof rollfilm backs are damn sweet, from my limited experience with them.
Horseman backs are nicely made as well. RB backs are nice, just be aware that the light-trap seals are generally quite worn, depending on the age/usage of the back itself. These are usually in need of replacement, which is quite easy to do. And since you're good with a cnc machine, I'd imagine installing some new foam wouldn't be too out of the ballpark either
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choose a format(2x3=6x9, 6x6/7/8/9) If I were you, I'd just get the 6x9, and crop it down to what you want when printing/scanning, etc.... Unless you like squares for everything, or the uncropped 6x7/8 frame.
-Dan