If you haven't made the purchase yet, my two cents is why not save a whole lot of cash and get the lens for the 645. That way you can play with that camera (I love the negatives a whole bunch more than any of my 35mm rangefinder or SLR stuff) for awhile and experience that system for awhile.
I understand it sounds like your having a very serious rangefinder gas attack (mine is fortunately controlled through medication for now), but I will repeat that I understand your love for the rangefinders, but I can almost guarrantee that you will start really appreciating your 645 if you use it more. That big, big negative is wonderful to work with, but I do love the pictures I can get out of my Lynx.......
Final thought, now that I have an operable rangefinder press camera......I really, REALLY want to step up to a 4x5.......perhaps you money will be better served by building it up for one of the larger format rangefinders......afterall, I think you can tell that everyone on this site has pretty much gone from 35mm whatever to a larger and larger format camera.
Good luck in your very hard decision.....been there, well about four days ago!!!