Your desired Renfe station is Barcelona Sants. If you press the '+' under opciones you get an itinerary.
Your time appreciation is way wrong. There is no way that you will make 2020 train following a 1915 airport arrival. It will take half an hour to get off the plane and collect your bags. Customs will take time too. Transfer from T1 to T2, from where the Cercanias train departs, every 5 minutes and takes 10 minutes. The Cercanias/Rodalies train from Barcelona airport T2 to Sants takes 20 minutes. and runs every half hour. You add this all up and you'll realize that you'll need more than an hour just to get clear of the airport. And you're making a very optimistic assumption that your plane arrives on time.
Yes you can purchase your ticket just prior to boarding. Beware there are often lines that seem to move like molasses.
Other options for travel to Sarria:
a. Fly from Barcelona to Santiago SCL. Catch Empresa Friere bus direct from SCL airport to Lugo. Catch Monbus bus from Lugo to Sarria.
b. Take Renfe train from Barcelona to Madrid. Take Renfe train from Madrid to Sarria. Advantage is more frequent service and therefore possibly faster. Note there are two stations in Madrid; Chamartin and Atocha. You may need to use Cercanias train to transfer from one to the other ... check the details.
Here is Aena Barcelona Airport
The official Barcelona-El Prat Josep Tarradellas airport website provides information on departures and arrivals, flights, car parks, VIP lounges, shops, etc.
www.aena.es
Here is Barcelona Sants
Administrador de Infraestructuras Ferroviarias (ADIF)
www.adif.es
Here is Barcelona Rodalies (train from airport to Sants)
Toda la información sobre horarios, tarifas, equipajes, mascotas y mapas de Rodalies de Catalunya - Renfe
www.renfe.com
Here is Renfe
Buy cheap train tickets AVE, Avlo high speed Low Cost ✓ Timetables, Fares and Discounts ✅ No Booking Fees! ▷ ( Renfe.com )
www.renfe.com
Here is Aena Santiago Airport
www.aena.es
Here is Empresa Friere
Here is Monbus
Buy online bus tickets and check schedules. If you prefer, hire a vehicle for your group travel. Travel in a secure, comfortable and cheap way with Monbus.
www.monbus.es