You've chosen a good time to get into DoK!
First up there's THREE new releases coming for them. This weekend there's a new Warcry starter set with two new warbands in them. These warbands operate in the main AoS
game and the DoK warband is looking very nice
Here's a heads up on their AoS
Suffice to say that if you don't want the boxed set there will likely be stock on ebay and forums from split sets so you might well be able to pick up the DoK team without much hassle. Right now they look like they'll be more of an elite group (the Slaves to Darkness Warbands which all the other Warcry unique bands are, are more chaff units taken in big blocks).
Then in November there's a duel Slaanesh and DoK boxed set going on sale with snakes, winged aleves and a brand new snake leader (not a character). If you like the snakes then that would be a sure choice to pick up; again either the whole set and then sell the half you don't want or swap the half for another player who wants more Slaanesh or find split kits on ebay.
Shadow and pain details - for the DoK that's 2 packs of snake ladies and 1 pack of winged along with the new leader (leader isn't a character so you can take more than one).
Finally there's a new campaign book "Morathi" coming in November as well. We don't yet know the full contents of this book but its bound to be good for the DoK. Again another thing to keep an eye out for
So its a busy time for the DoK and if you like snakes then the discount on a duel pack will be very welcome considering that DoK tend to be on the more expensive side for pure witch aelves.
After that the getting started set is fantastic value as the Blood Cauldron kit currently builds ALL the leaders for the army which are not either Morathi or the new Shadow and Pain leader. So at least one or two Getting Started sets will give you a good foundation of leaders and options.
Note that snakes you can take as battleline instead of witch aelves, but only if you take a Bloodwrack (either on its own or on a Shrine - both built from the Cauldron Kit) as your general. So if you do buy into Shadow and Pain I'd also get at least one Getting Started set to give you all you'd need. With that you'd be decently on your way to 1K points worth of models - esp if you take an Avatar from the Cauldron kit (since you'd built a shrine you'll have the avatar spare); and you'd have two queen models to use as well (hag and slaughter).