I think you'll be blown away by this game! It's terrific!
I bought it a few months ago, so I'm new, too. I pondered a lot before buying it, since i had never played ANY collectible or "living" card game (no, not even MagicTheGathering).
I can say I'm having a lot of fun with this game and (with very little effort!) I was able to involve a couple of other guys in playing it.
I actually own the core set, the "assault on ulthuan" expansion end the "march of the dead" expansion (this I bought only recently, having started with the other two). I plan to buy in a couple of months (probably) the "Legends" expansion.
I'm not at all into tournements and stuff like that, and none of my playing buddies are…we are (as far as "Warhammer: Invasion" goes) "casual gamers", in the sense that we're just willing to have fun and beat the crap out of each other with this game every time we can.
…so, having said that, I would like to give you my opinion:
It is a beautiful 1vs1 game: fast, brutal, smart.
It's NOT, in my opinion, necessary to buy expansions…maybe not even one: in effect it is already thoroughly satisfying with what comes out of the core box.
(If you and your gaming pal will like it, however, i would advise you to buy the "Assault on Ulthuan" set, which expands the game in many, many ways)
You will be able to play lots of different games without EVER having to "deckbuild". On the other hand, if you want to, you already have the means to start deckbuilding, but in a fun, user-friendly way.
Believe me: if you like playing (of course you are!), and if you like a good "1vs1" fantasy battle game, you are in for a good surprise!
After that, knowing that there are lots of good expansions for this game, isn't exactly like "having" to buy them, don't you think? Just remember that, if you finally get bored with the game you bought (and I think it will take a lot of time) IF you want, you can start looking into expansions, giving, i believe, eternal life to the game.
One last advice! Don't read too much on the W:I fans forum pages…almost 100% of the people you'll find have EVERY expansion, EVERY battle pack and OWN 3COPIES of the core set and the AoU expansion….just so they have EVERY CARD EVER PRINTED IN THREE COPIES… they might sound a bit "aloft" tho those, like me, who are just trying to have fun with the game.