Every hour there is a direct express train (RailJet, RJ or RJX) from Vienna International Airport to Wörgl, where you have one change to a local train (REX or S) for continuing to Kitzbühel.
The cheaper Sparschiene tickets you can get well in advance, but they are bound to the chosen train. If your flight is delayed and you miss the train, the tickets are void.
The regular price for this route is € 76, Sparschiene between € 60 and € 70. That price difference is not worth the risk of missing the train. (Significantly cheaper Sparschiene tickets you can get only for connections with 3 ore more changes.)
I would buy the tickets in advance online, including a seat reservation for the expected train to take. With a regular ticket you can take the next train if your flight is late; only the seat reservation is void then. Regular tickets can be cancelled until one day before departure.