Hearthfire was the perfect amount of base building for me. Maybe a bit more variation would be nice but i prefer the hand-crafted modular sections rather than having to construct every section of wall for myself.
Dawn guard had a good-ish story. It added new game play twists, and the vampire follower that you get is a blatant twist of Selene from Underworld, which I am perfectly okay with.
I got Hearthfire for $2.50, and it's pretty fun. However you will go through resources and gold very quick. However, I'm sure you will have a lot of that because skyrim is what, 5 years old now?