I just done my switch last two months - selling M8, CV 101 T, Rollei T, ... to Pentax 67.
I did try and still have my old Hassey lens bought last year with 4 lens. The issue with Hassey is that even if 2 Prism, it is not easy to handhold on eye level. I even got the grip .... but even with it, it is much worst than Pentax 67 in handling.
I ended with now 7 lens (55, 90 both LS and non-LS, 135, 165LS, 200, 300 nonEDIF) and NII plus nearly 3 extension tube, Helicord and reverse adapter. I even have the one for using my Leica/CV screw mount. So far very happy. Even using some of these lens with my D300 and it is okish, especially the 55F4.
I did miss leaf shutter lens as hand hold at 1/30 is a luck issue with 67 even with mirror lock ... I looked at Mamiya 67RZ, may be that is an even better option but once again the modular design frighten me after Hassey. Hence, no 1/15 like one taken with my Rollei T. Also, it is non-Zeiss, non-Leica, non-Rollei, non-..., only 1/30 sync speed for flash and it has no interchangable backs, ... But I think Pentax 67 is a good compromise especially NII Is such an advance camera - I like its spot meter, very good for my Velvia 50 picture taken.
The only one thing I really hate is it is very hard to load film to it. I started my film experience with medium format from 2004 with lots of these loading :-) Holga, Yaschica 124G, Bronica 6x6 and 645, 6x9 back on 4x5, Hassey, Rollei, ... a lot but nothing as bad as Pentax 67. I do not expect it would act like my autoload Fuji 645Zi but somehow I go through all the possible advice on the web and there is no good way to load it quick. I do not want to carry 2 body and someone said using 220 is hard on NII.
Hence, if you can survive the slowness to load film, I recommend 6x7 even if it is not a RF and full of limitations which I bet many of us can live with.
Pentax 67, Deadoroff 8x10, hessey 203fe, fuji 645zi
fuji gx680 "the breast", R3A/R4A and Nikon F4,
Yaschica 124G, Holga 66&612,
Tachihara 4x5, Kodak 2d 8x10
D: Nikon D300, Epson v700, Nex III&5n