Like previous posters, I own the 645, 645n, and 6x7. The original 645 is pretty cute with its 1980's Atari interface, but it's a very capable camera indeed. The only slight annoyance is that it gets 15 frames, compared to 16 with the later models. The 645n is a little sleeker to use, and has a layout that's a bit closer to modern cameras.
The 6x7 is a beast of a camera. It weighs a tonne, the controls are very similar to the original K mount cameras and earlier spotmatics. If you do go down this route, make sure the AE metered prism works (trust me, you want one with the light meter, rather than the standard prism). The 105mm f2.4 is a particularly fantastic lens, and the combination delivers the best picture quality I've ever seen. Other lenses (such as the 90mm f2.8, 75mm f4, 165mm f4, or 45mm f4.5) can be picked up very cheaply (the demand for 645 lenses is higher due to the 645z and 645d)
As great as the 6x7 is, I'd probably recommend the 645n with an FA or A 75mm f2.8. If you're on a tight budget, go for the 645 (although the loss of 1 frame per roll will start to eat into the price difference quite quickly). It's easier to use and isn't quite as expensive to run! Make sure the 645 you get comes with at least one 120 film back. Whilst some films are still available in 220 (only portra as far as I know?), you'll probably only end up using 120 film.
As for mounting 6x7 lenses on the 645, it can be done, but the adapters *can* be very expensive (there are some cheaper non-Pentax ones around, which you might be able to snag cheaply). Really though there is the slight humongous size factor to consider - the 67 lenses mounted on the 645 are not exactly portable