It's fun to see these Mamiya recollections here. My 500 DTL was my Dad's first camera and in turn became my first camera. I still use it for B&W work.
I have the 50mm f2 for it. It's an interesting lens. I've also owned the Sears 55mm f1.4 which is optically the same as the M/S 55mm f1.4 (both were made by Tomioka). It's on long term loan to the girlfriend right now. Optically, it was very close to my SMC Takumar 50mm f1.4. The Tak was stronger in some areas, the Sears in others. Both are very fine optics.
This was one of my favorites out of the 55mm:
Flickr Link
Congratulations on your purchase! I can't wait to see some images from it!