A big "thank you" to everyone who took the time to help me here. I really do appreciate it.
After carefully considering all the responses, and doing a fair amount of other research, I finally came to the conclusion that most of the systems have wides that would surpass my abilities. So I based the decision mostly on system and financial criterea. I decided that I want interchangable backs, which rules out the Pentax. Between the Mamiya and Bronica 645s, the main reasons were that the Bronica has the widest lens I can afford (since the Bronica 40 PE is wider than the Mamiya 45 and substantially cheaper than the Mamiya 35 N) and can accept standard cable releases without an additional cost adapter (all the Mamiyas that take interchangable backs use a proprietary electric cable release). I also like the idea of a leaf shutter, since with mirror up this means I can use a fairly lightweight tripod (Gitzo 2 series) while hiking without having to be concerned about vibration.
Between the Bronica ETRSi and SQ-A, the main reasons were price (the SQ-A with 40/4 would have been nearly twice as expensive) and because the 40 PS lens for the SQ-A takes 95mm filters, which would end up costing a stack (probably forcing me to buy into a filter system rather than being able to use circular filters). The 40 PE takes 62mm filters, the same as almost all the PE lenses, so I can use a set of relatively inexpensive circular filters. Between the Mamiya RB/RZ and the others, weight was the deciding factor - partly because my parents are in the USA for Christmas and have agreed to bring a camera back for me, saving a couple of hundred dollars in shipping costs, but I didn't feel I could lumber them with an RZ67!
So I've ordered a Bronica ETRSi with a waistlevel finder, 120 back and 40/4 PE. Total cost was US$ 510.
Thanks again for all your help.
Andrew