It Depends what DLC's you want, All DLC's or just map DLC's. If you want all of the map DLC's i recommend buying the Map Booster Bundle if you have no map extensions whatsoever. If you have some map extensions i would suggest just buying them separate as you wish. I don't think there is any way to buy every single DLC at once unless you find the multiple bundle options. My recommendation would be to buy what DLC's you want separate but use the bundles if you want all DLC's. To check you have all of the DLC's possible:
Go to Library
Hit Find more DLC's in store (seen in the image below)
Once you have clicked this, browse through the DLC's to check what is not in your Library.
I hope this helps ?