Trespasser is post-ending content and its meant to expand the Inquisition's story.
Jaws of Hakkon is either post or pre-ending, and it deals with past lore.
The Descent is either post or pre-ending, and it deals with dwarven lore. Only get The Descent if you like dwarven lore.
There are new schematics and stuff for every DLC, and so, if you want the maximum amount of content, get all.
Other non-story related DLCs that are SP DLCs are:
Spoils of the Avvar & Spoils of the Qunari. These are optional and they can feel like they're a waste of money. I suggest you look at review vids like for those two DLCs and see if you want them or not.