Great! I hope you'll have as much fun with it as I had (and have)!
About the two DLCs, well, you can decide if you want more once you finished the core game, which will take quite a while, believe me.
I've played them both and at 800 MS points (about 10€) I think the first one 'Reverie' is definitely worth it, whereas the second one 'Resurrection' was a bit of a letdown.
They're both quite short compared with how expansive the main game is, but Reverie has some really great puzzles and allows you to play for a few moments with a character other than Gabriel (I won't spoil who). I put the controller in the hands of my dad and my brother and let them solve some of the puzzles for themselves; they were really that much fun! (We all enjoy the Professor Layton games on the DS as well.)
Resurrection basically consists of one big boss fight drawn out over two levels. It's very hard, but adds little to the game. You can skip that one, but on the other hand, if you'll ever make it that far it might be neat to actually finish what you started (as it continues the Reverie story). You can of course always be on the lookout for a price reduction when there's sales going on during certain holidays or the summer.
So yeah, you can cross that bridge when you get there, but first enjoy the epic journey that will lead you to the Lords of Shadow!
