I've saved 700 gold, what should I buy?

People are telling me not to buy Naxx since it rotates out of standard soon, but I'm not sure what that means. So, BM wing, LoE wing, open packs? If I buy packs, should they be classic or something else? In Arena I average 3 wins due to a cracked screen causing me to attack my own minions and a serious disconnect problem, so I think that's out. Thanks for the help.