I have the newer versions (Super, and Pro) and prefer them to the older bodies due to handling and weight concerns.
If I were you, I would consider as well what sorts of accessories you might want. Some of the older versions are quite old, and items like metering finders may be harder to locate in working condition.
Of course, the big advantage of the Mamiya 645 system is that all manual focus lenses are fully functional with all manual focus bodies, and at least usable on the newer AF bodies.
Matt