This is mostly answered from the card text:
1. It tells you to search for, not name, a basic; so calling your shot is not part of the play (b/c this is Swoop! not HORSE :wink: ). Further, you must search before you discard. If you don't find a basic, you don't discard.
2. It doesn't say, "you may", so once you play the card, you're committed to do the discard of one of your basics in play IF you find any basic in your deck.