Crusader Kings 2 is a Grand Strategy game like no other. If you've played a Paradox game it is, in my opinion, the best out of all of them.
If you've never played a Grand Strategy before, know this, CK2 has no winning condition except for what you decide.
The game won't tell you that you have to reform the Roman Empire, or turn the Netherlands into a fortress-kingdom entirely owned by you, or do things that you definitely don't want to describe in public. All that is decided by you.
The DLC definitely improves the game, but is far from required.

Crusader Kings 2 is a Grand Strategy game like no other. If you've played a Paradox game it is, in my opinion, the best out of all of them. If you've never played a Grand Strategy before, know this, CK2 has no winning condition except for what you decide. The game won't tell you that you have to reform the Roman Empire, or turn the Netherlands into a fortress-kingdom entirely owned by you, or do things that you definitely don't want to describe in public. All that is decided by you. The DLC definitely improves the game, but is far from required.