As a happy owner of the Pentax 645n, do note that while the camera will furnish autofocus with autofocus lenses, those lenses are really expensive, probably driven by the 645 digital cameras, which can use them. Indeed, many of the manual focus 645 lenses have jumped in price after the digital 645 was introduced. Note that the 645n furnishes a focus indicator in the viewfinder, and an optional audio "beep." The manual focus lenses have smooth, easy action and are a pleasure to use..
I personally would avoid the original 645. Its user "interface" is clumsy, the viewfinder is dark by comparison with the 645n and 645nii, and it lacks the focus indication. The only substantive difference between the 645n and 645nii is the mirror lockup on the nii. According to The Luminous Landscape the lockup is really not need.