You may choose to find any kind of card, including a land. You must exile a card (unless the opponent’s library is empty); you can’t fail to find anything.

The exiled card is played using the normal rules for timing and priority for its card type, as well as any other applicable restrictions. You’ll have to pay its mana cost. The only thing that’s different is you’re casting it from the Exile zone.

“Until the beginning of your next upkeep” means the effect wears off as soon as your next upkeep starts. Your last chance to play the card is usually the End step of the player who immediately precedes you.