I have a Pentax 67, both the non-metered mirror lock-up model and the newer Pentax 67II. If you go with the pentax, get the 67II which has an excellent metering system including spot metering. While it is a beast, it is very well built. If you are trying to go light, the 67II with a 90 mm lens is not bad. I carry a 67II, 165 mm and 90 mm lens in an rounded top, rectangular (barn shaped) insulated lunch box with several rolls of film and a filter or two in the top. It is very manageable. From what I have seen, the 67II goes for about $1200 used which is quite a bit more than the regular 67MLU.

The Bronica GS-1 is lighter than the Pentax and the lenses are supposed to be top rate but good luck finding them. They do have interchangeable backs also, and the Pentax does not, but that added extra weight and bulk.

The 6x7 Fuji rangefinders might be a good bet for you in terms of lightness and convenience. But they are limited to a single focal length and are also a little harder to find.