Personally I would start at the beginning and work my way through. You can have a good amount of fun with just the core set in the beginning - for two players there's plenty of variety with the starter decks and then moving on to constructing "proper" 50 card decks from there. If you like it, then you can start getting expansions.
I think you'll also get more out of the adventure/deluxe packs if you purchase them in order - with power creep we're currently utterly annihilating everything pre-Khazad Dum with our Dwarrowdelf Cycle decks which is making once very fun quests rather trivial which is fine for us, as we experienced these adventures when they first came out, but I'm not sure how I'd feel if I'd bought an AP now and crushed it consistently with an unoptimised deck.
Plenty of other opinions out there though.
I think you'll also get more out of the adventure/deluxe packs if you purchase them in order - with power creep we're currently utterly annihilating everything pre-Khazad Dum with our Dwarrowdelf Cycle decks which is making once very fun quests rather trivial which is fine for us, as we experienced these adventures when they first came out, but I'm not sure how I'd feel if I'd bought an AP now and crushed it consistently with an unoptimised deck.
Plenty of other opinions out there though.