Lenses for Mamiya RB67 cameras come in three flavours: the old 'non-C', the newer C lenses and the latest K/L line. If you can afford it, go for the K/L lenses.
For portraits, I would recommend the K/L 180mm. It's a very, very good lens that can be picked up brand new for pennies these days. 180mm in 6x7 is more or less equivalent to 85mm in 135 (that's FX in digital).
For landscapes, the K/L 65mm (32mm equivalent) should be excellent. I have the older C version myself and it is a marvelous piece of glass.
ProSD backs work just fine on a ProS camera and are in my opinion well worth the upgrade. They have mechanical light baffles, which means no messing around with old light seals and stuff. Then again, that's just a personal preference.