I've found my Bronica SQ-B centred kit can get to around 20kg once camera rucksack and tripod have been taken into account. On the other hand, my "new" Mamiya 7II doesn't seem much larger or heavier than the Bronica RF645 it has replaced. However, on a note of caution, I often find the 250mm lens useful on the 6x6 SLR Bronica so I don't expect the Mamiya 7II to function as a direct replacement. The medium format SLR still has a place.
Are you planning to keep the Pentax 67 and do you use long lenses?