a) do you have other lenses that you might want to use with said filters? Are they hassy-bayonet or normal-screw?
b) do you have any filters currently, bayonet or screw?
c) what's your budget, and how easily do you get GAS?

If you've already got some or the other, it's probably going to be cheaper to use what you've got and build up the set from there.
If you're starting from scratch, the cheapest route would probably be going from hassy bayonet to regular-screw, and buying regular-screw filters that exist in every price-point from $5 to $500. It will also be easier to get cheap(er) nice-brand filters from fleabay in whatever size you need.
Plus, if you want to later go to Lee for their very nice 4x6 Grad NDs, you can then get either a bayonet OR a screw thread to Lee adapter.

I don't know about the hassy shades, unfortunately, but my guess is that they're bayonet-mount. Using step-up rings generally precludes the use of hoods on most of my other systems, so I'd be deciding on a hood system before filters (there's always cheap screw-in hoods that you can use if you get screw-filters available from fleabay but they're never as nice looking...)