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Good choice on the RZ. This is a great system, I shoot with is a lot, for all sorts of uses. I haven't had the 90 or 127 (though I think I have 14 or 16 RZ lenses), those are older lenses and likely superseded by the 110 (a great lens, the fastest and lightest of the system, and a must-have, IMNSHO).
the 65 M/L-A is absurdly sharp. a great environmental portrait lens. I'd get that and the 110 for a nice 2-lens kit. the 180 would be a nice choice for a portrait lens. The Tessar formula has great bokeh, and it's relatively light, and not very expensive. I find the 75 too close to the 110 to be all that different, thus the recommendation of the 65. The best all-around 75 is probably the 75/3.5L, which was discontinued fairly early on, and as a result somewhat rare and overpriced. The 75 shift or SB is a great lens too, but a bit more specialized (though it would work fine for full-body portraits, esp. the shift model.)
It's hard to go wrong with this system. I'd recommend a 20-20 brightscreen if you can find one. Also the Motor drive II and electronic L-grip help a lot for hand-holding. AE Prism II is a very nice addition too, esp. for hand-held shooting.
One of the great parts of the RZ system is it's so modular, plus there's a lot of it out there on the used market. It makes picking up and trying new lenses not that expensive. THey are also fairly easy to work on for DIY cleaning and repair. I've had several apart, for both reasons, and they've been pretty straightforward to work on. Mamiya USA is pretty helpful with spare parts and repair diagrams/manuals too.