1/500 is no problem.
New parts are not available for the older C lens shutters.
The newer lenses are a little more user friendly.
There are many threads here that cover the differences and the pros and cons of the various versions.
See if you can try one out at a store or photo show, or borrow one, and see how you like it.

The Mamiyas are fine cameras, and have no appologies to make to Zeiss optically, but they are quite a lot bigger and heavier.