It's a wonderful camera system although it weighs a ton. The lenses are superb. I checked the shutter speeds recently and they were quite close to correct, excellent matter of fact and these are 30 years old.
I would go for the 65 and 180mm if I had to pick just two of them. Probably Sekor C or K/L.
Just a personal preference for focal length, I think all Mamiya lenses are manufactured with the same high standard. They are all great lenses. Really great!
Luckily for us who uses the system, the prices are very low (dirt cheap!) compared to Hasselblad, Mamiya 6, Mamiya 7, Pentax 6x7, and other "cool cameras".