“One for One” does not target. The monster you Special Summon is chosen at resolution. If you were planning to Special Summon a Level 1 monster from your hand, but no longer have one at effect resolution, you have to Special Summon from your Deck instead (and vice versa).
If there are no Level 1 monsters in your hand or Deck, you cannot activate “One for One.”
Sending one monster from the hand to the Graveyard is a cost to activate this card.