I have owned the Pentax 645 and the N and Nll.
You want the N or Nll. The original camera has a goofy electronic shutter speed button selector and a dimmer viewfinder plus it is strictly manual focus.
I owned the Nll because a seller on ebay advertised it as an N so I bought it at the N price. The big deal about the Nll is the mirror lock-up. The mirror on the N is so well dampened that this feature is not necessary. I liked the textured finish on the Nll. Not as pretty as the N but did not show scratches in the long run. The real advantage of the Nll is that it is newer.
The lenses to buy are the 150mm, 75mm and the 35mm. The macro lens is also nice (I forget the focal length). Buy the 75mm and 150m with autofocus. Save your money and buy the macro and 35mm with manual focus.
No, the lenses are not Zeiss sharp but they are plenty sharp for 8x10's and 11x14's. Of course if you want ultimate sharpness in medium format you should be shooting a Mamiya 7.
The Pentax N and Nll are really fun cameras to use. With autofocus and matrix metering they are as easy to use as a digital camera but have that wonderful film look. With their huge viewfinders they are a pleasure to use handheld.
If you are interested in the 645 format then I highly recommend them.