You should specify that even getting all the current deluxe expansions/mythos packs won't be enough to equal a second core set in terms of deck customization. Though being a LCG, all the extra stuff helps in expanding from the core set, it's not yet at the point in its life where just getting the extra packs/sets is sufficient.
I modified the OP with this text. Hopefully that helps out people who still have this question a little more.
You can play and enjoy the game with a single core set, although I would suggest playing on Easy difficulty. Buying the expansions will give you more versatility in deck customization, but will not equal what you have by buying a second core set. The core has many staple cards that do not have equivalent cards in the expansions, so having two copies of these is going to make your decks more consistent.