Hello Caurelloo90
I suggest to buy a Swiss Half Fare Card. With this card you can buy half fare tickets from and to the airport and to go to the Jungfraujoch (Top of Europe):
https://www.sbb.ch/en/tickets-offers/tickets/guests-abroad/swiss-halffare-card.html
It is not recommended to buy the tickets for the Jungfraujoch in advance, wait to know when the weather is good.
From and to the airport you can buy Saver Day Passes with the reduced price if you have the Half Fare Card:
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html
The two first trains have 1st class (from the airport to Lauterbrunnen), you are more as 2 hours in theses trains. Only the last leg Lauterbrunnen - Wengen has only 2nd class (13 minutes).
Kind regards
Danhiel