But why do you recommend do buy GH or SotF first?
SotF adds nice new stuff like barricades, unique items so adds some new twists.
GH adds lots more of the same stuff giving you more locations and heroes.
Both give you new hero/zombie cards.
BitF gives you more space to play on (the forest, obviously), some nice new zombies and the experience thing but, IMO, it's not nearly as game-enhancing as the others.