Well, after much research and soul-searching, I pulled the trigger on a Mamiya 645 Pro.
I ruled out rangerfinders (for now) and I really wanted to like the Pentax 645 or 67 but they didn't quite fit the bill. I also like the Mamiya RZ67 but I was scared away by the beastliness of it. Finally settled on the Mamiya 645 Pro which seems to offer much of what the RZ does in a slightly smaller, less cumbersome--and most importantly--less costly package.
Thanks to everyone for the advice. I'm excited to shoot my first roll of 120 film!