Keith
The best camera is one that still works!
I can only support Dexter's assessment of the Mamiya 6 75mm lens bokeh. I would only add nearly unusable meter with the 150 lens (which has a very nice boken in contrast to 75) as the Mamiya 6 meters area larger that the coverage of the 50 lens.
I would say - if you like the RF645 - hang on it and be patient to get the 135 lens - and sell the 100. As both of these are rare you will not loose much cash in the proces.
I am actually thinking of the RF to replace my Mamiya 6 (I have all 3 lenses) - I just got a Minolta Autocord for 6x6 (I love this format and Tessar lenses).
To get an idea of how much bigger the camera gets once you go from MF to SLR in 645 format look HERE (side note - I would LOVE to get my hands on both RF645 and Contax 645)
I'm starting to feel like it would be very hard to get rid of the RF645 ... after putting a roll through it the other day I was reminded just how good it is and how compact it is. It seems a little fragile and erratic at times but in terms of usability it can't be faulted IMO.
But ... I don't recall seeing the 135mm lens for sale anywhere ... ever!