I'm planning on pre-purchasing Destiny 2: Digital Deluxe Edition, so does anyone know if that includes the base game or do I have to buy it separately? If so, where can I buy the base game?
You do not need to purchase anything beyond the Shadowkeep expansion (Standard or Digital Deluxe Edition). The first year of Destiny 2 content has been renamed to New Light and made free to play on all platforms. You can read more about New Light here:
https://www.bungie.net/7/en/Destiny/NewLight
Shadowkeep will sit on top of New Light. The previous expansion, called Forsaken, also sits on top of New Light and is $25 USD. Forsaken is not required to play Shadowkeep.
destiny 2 digital deluxe includes all current game content. it does not include shadowkeep.
shadowkeep digital deluxe would include all current game content EXCEPT forsaken which would need to be purchases separately but that purchase includes the forsaken annual pass (which is 'free' to everyone with forsaken now).