My wife has a 645, and I have both the 645n and the 645nii. Frankly, they are all excellent cameras -- very durable, quite ergonomic and wonderful to use. I bought the 645n only because I liked the metering and the more traditional shutter speed dial. And I wanted a second body, so I went looking for another 645n but stumbled across a really good deal on eBay on a 645nii and got that, instead. That said, I see no real advantage whatsoever of the 645nii's features over that of the 645n -- certainly not enough to warrant a significant price disparity. As others have noted, the mirror lock-up is really not needed at all with the 645 series cameras unless you intend to shoot with very long lenses and/or very low light.
As for the 120 v. 220 film holder -- you will pay more for a 120, likely, but you need it. It is virtually impossible to find 220 film rolls anymore, and quite expensive to acquire the necessary reels for it (if you intend to process your own film ever). Conversely, the 120 film is quite easily gotten.
You are, in my opinion, entering into the very best Pentax camera series with the 645. Congratulations!!