yoy need the right adapter ring and the fiter holder. For 24mm the wide angle adapter ring is recommended, albeit it is more expensive. The normal one might cause some vignetting. The standard Lee filter holder will do fine for two or even three filters. If you want to use the square filters and a pol filter, you will need the Lee adapter ring and longer screws to put in front of the square filters a 105mm filter. Sigma and B+W make such large pol filters.
The adpter rings, filter holder and pol filter can easily cost you more than $500.
The Lee filters should be quite good but they are hard to get sometimes. The filters by Hitech are a good alternative and especially the sets with a size of just 125mm x 100mm are much cheaper than the 150mm x 100mm Lee filters. The 25mm more length of the standard grad ND filters you will very seldom need (never happened to me).
I do have the wide angle adapter rings in varius diameters, the Lee filter holder, 105mm adapter ring, Sigma pol filter and 10 different Hitech filters in a Lee filter case, which cost me all together more than EUR900. I don't want to miss them anymore. But I think you need to do landscape photography pretty seriously to invest such a lot of money and also time to get these filters.
Kind regards, Robert