I had Pentax 67s on two occasions. The first time, i had a II that i used for film and a MLU that i dedicated to a Polaroid back. Both were used only 'in-studio.' I had a VERY strong preference for handling the 'II.' The grip is a tremendous improvement over what i considered "Flintstones-like" design of the older models. The II just feels significantly more refined, even though neither of them is refined in the way of a Contax 645.
Don't know about reliability. I would suggest buying one in the best condition possible, as any that have been used by pro fashion photographers will have been used and abused. With the older models, the scuttlebutt was that the winding mechanisms were either 'weak' or prone to need repairs - but possibly only due to the aforementioned fashion fotogs using them in rapid-fire mode.
With regard to cost, for me, i would only consider buying a II, as i really disliked using the older ones. They just felt too primitive and rough-hewn. But, many people feel differently, as even when the II was released, guys like Bruce Weber, Steven Meisel and Mario Testino continued to use the older models. They seem to like using them with those huge wooden handles- which i never understood, since they sit on the left side, occupying the hand you need to focus.... Maybe it's not so much a grip, but something to hold onto when you're not shooting it? I dunno....