The standard suggestion is to check out other expansions, since they give you the best cost per card ratio.=) March of the Damned and Legends give you a very good fresh start plus they have some veery good cards. Another good thing about them is that both of them have three copies of all cards.
On top of that, the most recent packs cycle has 6 BPs centered around different factions (1 per faction). E.g., "The Inevitable CIty" makes a corrupted Chaos deck possible through such cards as Bleeding Wall, Khorvak (Hero that allows corrupted units to attack from any zone) + other cards that benifit from corruption.
So March of the Damned + Legends will give you 165+165 cards = 330 (55+55=110 different ones) at a price of ~$50-60 (depends on which store you try to buy them at). Next step depends on which faction you'll go for above that, but I'd start by buying these 2 expansions.