For a 6x6 SLR, I'd recommend a Bronica SQ-A or SQ-Ai. Great lenses, especially the later PS versions, and fairly hand-holdable with waist-level finder and a speed grip. You'll still have the loud thunk from the mirror/shutter that most medium format SLR's have. I have an SQ-A that's been chugging away with no problems for over 15 years. Very versatile system. I would also consider Hasselblad, if money was no object.
Don't overlook Rolleicords, if you want a TLR with the same build quality as a Rolleiflex, but less cost. If you can live with a one-lens camera, these are fairly light (less than a 'Flex, much less than a Mamiya C3, C330, C220, etc.). Schneider and Zeiss lenses on the Rolleis are really amazing.
Folders are also an option, Zeiss Super Ikonta or Voigtlander, a few others makes as well. Condition of the bellows and struts is important, maybe check out a seller like Cert06 (Jurgen) and see what he has to offer.