If you go the Cat route, make sure it comes with it's filters. At least in the case of the Nikkor's, the filter is a required part of the optics.
As I recall, the lens was supplied with a clear filter and some ND's.

Unless it's stuff you already have, I'd avoid a teleconverter with a zoom, if you're concerned about sharpness. The result probably isn't any better than a Cat prime, and might well be worse, depending on the specific lenses you'd be comparing.

The nice thing about Cats is the lighness. Hand holding a straight tele of 300 and longer is challenging without extra support of some kind, but a 500 mm cat is much easier to handle, and realistically do-able without extra stuff.