I was wondering if there would be any benefit to getting it while I am lower level, or should I wait until I am high enough level to go to the new area? For instance does it add new items/skills/etc. to the base world, or must you go to the expansion area to have any of the additions?

Well the armour you can craft requires materials from Solsthiem (Stalhrim, Chitin, etc). You can make the Nordic Curved set and weapons though, as it requires only materials you can find on mainland. Beyond that I don't know.

Ok, thanks!

Theres a new type of draugr which spawns in dungeons known as 'hulking draugr'. Yet it dosent seem to have any solthsteim specific weapons or equipment from what i could tell.

Last Vigil is added to the main world.

It always existed. But now it's a landmark.