If you're thinking of medium format rangefinders, the Mamiya 6 or Mamiya 7 are great choices, but with the caveat of limited close focus, as with most rangefinders. This isn't so terrible on the wider lenses, but with the 150mm lens, your minimum focus distance is 6 feet, which is not close enough for a tight head shot. If you think along these lines, look at the Mamiya 7 instead - there are five lenses available for it, instead of three, and it gives you the 6x7 aspect ratio which you can always crop if you want back to square. With any medium format rangefinder you're not going to get the fast lenses you have with 35mm. Why not just stick to your Rolleiflex? There's no reason you can't use it for family photos as well as work.