I'm planning on getting this as a game to play at home with mates, I'm not really a tournament player.

So, I'm wondering if there's a sensible way of buying the chapter packs that means I could build more decks (or expand existing decks) without one becoming more powerful than all the others.

For example just buying a core set + wolves of the North, I assume would make the stark deck more powerful than all the others buy way of a larger card pool.

What's the best way to avoid that?

More cards doesn't necessarily mean that the cards are good.

The big balance-shifter in WotN is Winterfell, which shuts down the use of player triggers for an entire challenge (usually Intrigue on defense) unless there is a Winter plot revealed. The easiest counter is to make sure every other deck you build for your group includes one or more Winter plots (The First Snow of Winter (No Middle Ground) is particularly good for this, but presents its own issues).

Welcome!

First, you should know that Core set does include 211 distinct cards, coming from all 8 factions, but most of those cards are only a single copy. So the best purchase you can make after your first Core set is another Core set.

As far as balance goes, there's no easy route. Just tweak the decks every couple games if you find that one is too powerful relative to the others.

Yeah, the first deluxe is only the third best expansion after a second core, then another core

Wolves of the North definitely gives Stark new toys, and a wider variety of deckbuilding options. But I don't think there are any "I WIN!" cards you can't counter with what's released so far. Even the classics from the core are still sometimes fine. So just pad the rest of the factions with more core!

Yep, second core is the way to go.

I have four decks for friends to play and I used 3 cores to make those decks. The only problem with that is that those decks are not fast enough, so the games tend to last a little bit longer. I suppose that i Have to buy 3 cores more, to make 4 very fast decks...

But 3 cores is ok. After that i would buy deluxe to each faction, just for the deck building options. But if you are not going to do deck building. 3 Cores is the way to go. Very little deck building. Relative balanced decks. The Lannister is good and Nightwatch not so, but otherwise the easiest way to go.