The rulebook says that in order to purchase a level 2 card, you must first have two level 1 cards in your deck.

Can I purchase all three cards at the same time, or must I first purchase the two level 1s, complete a quest, and then purchase the level 2?

Dont see why you couldnt purchase all 3 at the same time

My understanding of this would be (assuming you have the 4 XP points required) that in the same campaign phase:

You buy the first level 1 card, you now have 1 level 1 card in your deck.

You then buy a second level 1 card, you now have 2 level 1 cards in your deck.

You can therefore then buy 1 level 2 card to add to your deck.

The only thing to bear in mind is the two level 1 cards must belong to the same class as the level 2 card you want to buy.

You can "bank" your experience for as long as you want to. So if you have four experience, by all means you can buy all your cards at once. Most people would say that it's best to spend what you get as soon as you get it, but I have banked for a quest or two to see how my heroes are developing before I decide how I will retaliate. It's a sound strategy.