Kreuzberg is a big area and the quickest route depends on your starting destination. I'd advise you to check the best route for yourself here https://www.bvg.de/en but the SBahn that goes to Schönefeld is the S9.
Buying the 72h version is up to you and your need for public transport. When you take a subway or bus more than 5 times a day, it is already cheaper to get the 72h ticket. Also, the 72h ticket gives you a discount on entry fees for many museums.
Schönefeld is in the C-Area of Berlin's public transport provider BVG, so you should buy the ABC-version, of the tourist ticket, even though you will need it only for the fare to Schönefeld.