a somewhat rambling reply
I'll supply a bit of background about myself. I was once a pretty serious photography student. I have quite a bit of experience with medium and large format shooting. I'm also an avid hiker. I spent the summer fo '99 at the maine phtographic workshops where I exposed a lot of film. At the time I had Mamiya 645pro system. I carried that thing around a lot and liked it most of the time. mid way through the summer I had the chance to "borrow" a mamiya 6mf for a day. It changed my whole way of thinking. I loved it, and it is still my favorite camera ever. I sold the 645pro gear and picked up my own 6 with 2 lenses later on that year.
shooting square versus rectangle is an easily surmounted obstacle. If you have any tlr experience it will be an even smoother transition.
fast forward 13 years, and now I'm looking for the same thing as our thread starter. money is an object, otherwise I'd have a mamiya 6/6mf again.
Ultimately, for the way i shoot, I need multiple lens options. i really like the Fuji rangefinders, but I would be found wanting when in the field. Also, there are some really nice old folders out there, but still fixed lens. In my research, it seems like there is no wide angle option on any of the older folders; (ikon, ikonta, mamiya6, voigtlander bessa, perkeo, etc), unless I've missed something.
( I have zero experiece with the newer RF670, or the RF645 although both seem like they could be fine cameras)
If weight is the ultimate issue, and you're willing to live with some electronics, I'd say go for Fuji GA or GS cameras ( obviously GA means more elcronics) Then the Mamiya rangefinders, (Plaubel Makinas would mean more than one body), then a modular system ala hassy, Mamiya, Bronica, etc.
If zero electronics is the drving factor then you're into older folders TLR's, and some system cameras.
money; Lately, the cheapest Plaubel Makina 67 series I've seen was 1200. i've seen a supposedly working Mamiya 7 body for 500, Hasselblad stuff is all over the map.
lens sharpness; imho, this is beating a dead horse, and is a somewhat subjective issue. So not taking into account exacting tests done in a lab type atmosphere, if you are a competent photographer and understand basic exposure and depth of field principles, you can make a pretty damn sharp image with almost anything, and a pleasing image with even crazier things (Diana, Holga, star filters, etc etc) Conversely, if you have no medium format experience, your image quality will probably be the same whether you spend thousands on a Hasselblad full system, or hundreds on an old Mamiya or Bronica system.