Heya,

Let's assume that I don't care for the Premium item and the panda, don't like "endless dungeons" but want the rest of the content, what should I buy ? I am confused by all the stuff.

I guess main game + season one pass cover everything ?

The red panda and the "endless dungeon" become parts of the main campaign if you buy them. The pet adds skills to its owner, and the dungeon adds a separate quest to the main campaign.
So if you want to have complete experience of the game then buy everything. Or stick to the main story if you want.