Who is the Main Villain in Hearthstone’s Curse of Naxxramas

No, you'll just earn 6 cards (1 legendary and 5 'commons').

Cards you'll earn :

Haunted Creeper good for a zoo deck (it's a deck with low cost cards and very agressive) you'll earn this if you beat Anub'Rekhan in normal.

Nerub'ar Weblord good for counter battlecry, you'll earn this if you beat Faerlina in normal.

Nerubian Egg it'll combo with a solfurie or an Argus, because if you destroy it, you'll earn a nerubian (it's a 4/4 cards), you'll earn this if you beat Maexxna in normal.

Maexxna she destroy any card she attacked, it can be usefull if you play Anduin Wrynn because he can heal Maexxna. You'll earn this if you beat all the 3 boss in normal and you'll unlock heroic and druid and rogue class challenges.

Poison Seeds : destroy all monsters on the battlefield and give treants 2/2 for each destroyed monster. Can combo with a druid token. Unlock when you finish the druid class challenge.

Anub'ar Ambusher a 5/5 for 4 mana but with a Deathrattle (Return a random friendly monster in your hand) can combo with Si 7. Unlock when you finish the rogue class challenge.

You can find many others information on the wiki : http://hearthstone.gamepedia.com/Curse_of_Naxxramas

The shop got overloaded when everybody tried to buy it at midnight. It's been going up and down throughout the day.

Real money purchases are fine- at least I haven't seen anyone complaining- because the real money purchasing system is so much more robust. This is understandable- charging someone real money twice would result in a lot of hassle and potentially legal action whereas Blizz can do whatever they want with Gold.

Gold purchases are still glitchy at the moment, judging by the forums. You're probably better off waiting.