The Bronicae that you mention are stylin' and, if working examples, can be fun. The model C might paradoxically cost more despite being (for lack of interchangeable backs) less flexible than the S/S2/S2a. I suspect there are fewer C units, so Bronica completionist collectors might bid those up. Numerous articles around the web will tell you of well-known problem areas on the S/S2/S2a of which to beware, so heed those. My S2 (sorry, not for sale ) required only minor focusing screen adjustment. Once you get the camera going, it can be fun, and a conversation starter, and beautiful, but frankly isn't the most practical shooter. It's more of a luxury for occasional, contemplative use than a high-volume daily user -- by which I mean no insult; that could be exactly what makes a good birthday present, especially if by chance your gf is someone who always chooses practical, proletarian things for herself.
BTW, what fun is life if you "remove the temptation of buying more lenses"?!?! Who wouldn't want more beautiful, chrome-nosed, heavy, old-school Nikkor glass?