IF CK2 is any indication, expect quite a few of those gameplay DLC's for EU4.

Just grab the paradox games only in sales like the one on Bundle Stars or when massively discounted.

Personally the easiest one for me to get in to Paradox games was Hearts of Iron 2 Complete. Very understandable and good UI. Crusader Kings 2 is a very good game but still a bit more complex especially with all the family stuff..

Victoria 2 can be very hard and demanding since you have to develop the industrial base of the nation, do the research and create armies. Its basically a game of balance and a time fight against the english empire which more or less leads the game especially if you play european countries.