I have recently bought the Core set and the KoTS to build a simple Greyjoy deck for myself but I want to one more similar(balance-wise) deck for house lannister or targaryen. Could someone please suggest something? Thank you in advance

Well, the Targaryen's have a "deluxe" expansion available to them, Queen of Dragons. Unfortunately there is not a Lannister deluxe expansion available as of yet. So if you're only looking to pick up one more thing, I'd probably go with Queen of Dragons.

if you want a quick balanced deck go for QoD with you are almost complete . Lannisters need almost every chapter pack of king's Landing to build a good deck .