I heartily recommend the P67. Its principal strength, for me at least, is the glass...superb! I have three lenses (55, 105, and 200) and that group covers my needs completely. The other big advantage of the Pentax is depth of field preview, and ND grad or polarizing filter adjustment. The Mamiya just won't allow that. I still have my C220 with three lenses that I bought years before the Pentax and I have great respect for that camera, but it just doesn't really rival the Pentax. Though having an interchangable back is indeed a great strength with an RB, RZ or a Hassie, the price of a used second P67 body is comparable to the cost of the film backs for the others, so you can accompish the same thing a different way. And the lenses for the Pentax are far less expensive since they don't include a shutter.
Regarding going right to 4x5....well, again it's the glass. I can't use my 4x5 enough...just love it!!, but the lone Geronar 210 lens I have just isn't the equal of the 6x7 lenses. And that's the rub for now....though the cameras can be purchased without ransoming your children, really decent lenses will separate you from...well...at least any purebred pets you might have.
so....stop eating for a while...walk everywhere...stop gambling and give up alcohol....the Pentax will be yours by the time your liver heals ;-)!!