Well unlike Alan (
@woof woof) above I regularly use filters when I am shooting seascapes/landscapes etc
wouldn't do to all be the same. I agree with his "new toy danger" comment though, it happens
As you ahev already worked out yur lenses have a 77mm and 82mm filter thread. You will definitely need an uwa adapter for your 16-35 otherwise you will get vignetting at the wider focal lengths. You may get away with a standard adapter for ypur 24-70, but tbh it is a realtively small one off cost and I'd get an uwa adapter for that too.
I've used a company called Speed Graphic a few times and they always seem reasonable, but worth shopping around too. Also there are often filter bits in the classifieds here, and sometimes for a decent price too!
So, the 77mm & 82mm adapter rinsg I'd suggest is this or equivalent
https://www.speedgraphic.co.uk/holders__adapters/lee_100mm_adapter_ring_wideangle/12306_p.html - you need to choose the size from the selection drop down.
These screw onto the front of the lens, and the holder then clips to this.
This is the holder you need
https://www.speedgraphic.co.uk/holders__adapters/lee_100mm_foundation_kit/6831_p.html (hint - there are two for sale in the classifieds now - not mine though
)
In the Lee system you have 3 choices of ND filter starting with the most expensive (and imo the best) and these are the pro glass filters
https://www.speedgraphic.co.uk/neutral_density/lee_100mm_proglass_ir_nd_2_stops/27417_p.html (eyewateringly expensive though) - next are the resin filters
https://www.speedgraphic.co.uk/neutral_density/lee_100mm_neutral_density_resin/12342_p.html (very good and still not cheap) or the polyester based filters
https://www.speedgraphic.co.uk/neut...tral_density_filter_set_polyester/6890_p.html (I have never tried these so cannot comment on usability, be aware they will damage easily but they are cheap and you get a set of 3 for less than a resin filter).
After tha it is up to you if you want/need a polariser (oneof my most used filters, particularly for lakes/waterfalls/wooded scenes). Be warned you may need a second mortgage for the Lee ones
you would need the ring and then the polariser which is this
https://www.speedgraphic.co.uk/polarising/lee_100mm_landscape_polariser__adapter_ring/26154_p.html
If you are ever in north Devon, let me know and I'll happily take you out and show you the ropes with these.
There are other brands available, but I have little experience with them, but look at cokin, formatt-hitech and progrey (I think the latter may be more expensive than Lee but ...) - so may be worth doing a bit of research. At least you should have the basis for a comparison now
And remember, check the classifieds here