Pick by what you do in a group: tank if you hold the front, healer if you keep people alive, DPS if you deal damage.
On a new character, start with the leveling pack and add a role pack as you near 60.
You can combine packs. Shared addons use the same folder and do not clash.
A pack gathers enough addons to get started on one job; it is not a complete UI setup. You can combine packs. An addon that appears in two packs is the same folder, so installing it twice changes nothing.
The addons in a pack zip are byte-for-byte the same as the single downloads. Each addon's author and license are listed on its own page and in the PAKET-ICERIGI.txt file inside the zip.