Since nobody answered your question, i assume we don't have much landscape shooters here in this subforum...

I'm a Lee user myself, but had used the Cokin before this.
Lee is better in terms of Quality, but the Cokin is quite good when it comes to price to performance ratio.

Here's what you need:
A fliter holder, with the adaptor ring (Sometimes comes seperately)
Normal Cokin Holder+Adaptor Ring
Wide Angle Holder
Then get your own desired filters and you have it all...

You can also consider to get the kit straight away which has everything except for the adapter ring iirc.
Cokin Kit

Should you decide to go with Lee, do bear in mind that there's a very long waiting period out there for the filters and the price of 1 Lee Filter would have got you a whole Cokin Filter Kit with money to spare. But quality wise, no doubt Lee will be way better

Yes, the holder is universal and supports most lenses with a filter thread up to 77mm. For lenses like the Nikkor 14-24 F/2.8, you'd need to get a special holder from Lee (SW150). What makes the difference is the ring, and yes you will need a ring for all sizes. What a friend of mine does is to get a 77mm ring and to get step down rings. This way is cheaper, but might cause vignette if you do stack too many stop down rings.

As for the coloured filters, the most common one i see people using is the Tobacco, which is orange in colour. The other colours which you mentioned, i have not tried, but you can always try them out, no harm. Especially if they don't suit colour photography, you can use them in black and white photos. But well, colours can very much be simulated in Photoshop nowadays.The must have filters will be the Gradual NDs, at least get a 3 stop Hard GND and another 3 stop soft GND. That will give you quite a lot of flexibility.