All of the community stuff is free. LW doesn't have any official WotC add-ons to purchase. Kobold Press has the most purchasable stuff available.
The community pack has the Oath of the Crown included in the SCAG stuff. When you download the community pack, you gain an expanded character options menu where you need to check the checkbox next to the community packs you want to enable on a character. You see these options when creating a new character or when you hit Ctrl-K on an existing character.
I think daplunk hasn't updated the post you've used for research, the newest notes for the community pack are often found
here.
I don't think the community can support the orders of knighthood beyond providing the name as an existing "membership" choice, most of the "flair" can't be replicated without LW getting a full WotC blessing. There is a spot on the Background tab where you can add a custom membership though.