You do not even need the kit, just the 4x6 filters. You can use the polarizer you are using now, hand hold the 4x6. Just a suggestion for starting out inexpensive.


17mm would be to wide for my taste, just a personal preference go with the 24mm.
I bought the Lee, at the time you couldn't find the Cokin holder.
The Lee works fine. Hand holding is faster.
There is no one "right" filter. 0.6 may be right one time, not the next. I suggest you get several, 0.6, 0.9 etc then you can stack them and have a more diverse effect.
Hard vs Soft, I find more use for the soft. But they are somewhat different in their use. There is not a incorrect answer on this one.