I’ve long thought that two expansions good for new players are Foundations of Stone and The Steward’s Fear. Admittedly, you can’t play the scenarios without other purchases, but the player cards in these sets really upgrade deck strength for new players with a minimal investment.

To test my theory, I built a deck with only the core set and these two expansions:

http://www.ringsdb.com/decklist/view/9616/a-potentially-strong-minimum-purchase-deck-1.0