Get Naxxramas. Second wing has Loatheb, the most used legendary of Naxxramas, and actually full Naxx is the way to go. You can try playing Arena when you would get bored on hoarding gold for Naxx, consider these as "one pack for 150 gold" so don't spend all you've acquired from other sources. Playing Arena would also help you understand card mechanics better, and thus prepare you for constructed play or even infinite arena play (you can win more than you've invested on arena, but it's very hard).
Naxx until the 4t quarter, fifth is not that necessary, then first quarter of blackrock mountain and congratulations!! you have patron warrior, no really the deck mostly uses basic set and adventure set cards