Getting back in to Witcher 3, have some questions

Took like a 5 month break but just came back. First off, DAMN the UI looks so much better. Spoiler: I just reassembled the lodge of sorceresses and was wondering how much longer the main story is? Does the game end after I finish or can I keep exploring? I want to buy both DLC's because I heard they were fantastic, how does the main story progress effect the DLC? Is there a level requirement? (I'm 30?). Can I start a new game and jump right into the DLC?