Which Dominion Expansion Should we Buy for Passive Play?
From a pure numbers standpoint, Dark Ages would be the best with 37 new Kingdom cards, only 6 of which are Attack cards. From how well the sets are regarded by the designer (and others from what I have heard), you are better off getting Intrigue or Seaside.

Q: If you could go back in time and redesign which cards were in which sets, and the order the sets came out (but not changing/adding/removing any cards), what would you move and why?
Vaccarino: For set order I like going Intrigue, Seaside, Prosperity – set trying to be a good first set, generally good set, set that adds Colonies. I would put Alchemy last, where it was; it was there because I knew some people wouldn’t like potions, because somebody hadn’t. You could swap Hinterlands with Seaside but I wouldn’t. After Prosperity then there are Hinterlands and Dark Ages to order and well you have the question from before of whether or not Hinterlands is a standalone. I like having another standalone, and if it is one then I would put it ahead of Dark Ages. Possibly I would anyway. A question is, do you recombine Cornucopia and Alchemy. If people don’t like Alchemy then it’s nice that they get a tight package of just it, rather than buying it to get Cornucopia or passing on Cornucopia because of it. So possibly it’s worth keeping them separate. Large sets are better though, so either expand Cornucopia and Guilds or combine them. That’s another decision to make before knowing what order to put them in, but Guilds wants to be near the end due to complexity. If they’re not one set then large Cornucopia could go between Prosperity and Hinterlands still, uh depending on how it turned out.

-Intrigue - 21/24 Non-Attack cards. Intrigue adds Dual-Type cards. From a ratings standpoint, I believe the highest rated sets are
-Seaside 21/26 Non-Attack cards. Seaside adds duration cards.
-Alchemy 11/13 Non-Attack cards. Alchemy adds a new Potion treasure.
-Prosperity 24/27 Non-Attack cards. Prosperity adds new treasure kingdom cards, VP tokens, and high cost Platinum and Colony cards.
-Cornucopia 10/13 Non-Attack cards. Cornucopia rewards players with diverse decks, and offers 5 Unique Gifts available in the Tournament.
-Hinterlands 23/26 Non-Attack cards. Hinterlands adds cards that have an effect when bought/gained.
-Dark Ages 31/37 Non-Attack cards. Cards that do something when Trashed, care about the Trash, or are upgradable.
-Guilds 11/13 Non-Attack cards. Coin tokens that can be played on later turns, bonus effects for paying extra for cards.