RZ67 and Hasselblad 500/501/503 series are the only options I would consider for a modular medium format studio camera. I hate the 645 format and find 645 SLRs to be too similar in weight and bulk to 6x6 cameras. I use a pair of Hasselblads -constantly- for what it's worth. I also use a Pentax 67, and love it, but find it less of a studio camera (it begs to be outside).
You can't go wrong with an RZ if you want a 6x7 modular studio camera though, they're pretty much the defacto standard definition for "6x7 modular studio camera". My father has used them for 20 years, formerly with cases upon cases of Portra and Tri-X, now with a Leaf back.