I plan to get a Raspberry PI 2 Model B and the following touchscreen http://www.amazon.de/dp/B00LN9MYCO/
Now my question is, which OS or desktop environment can you recommend with good touch support?
It's my understanding that Raspberry Pi's of all flavors can run Android. If you want a touch aware OS, that's probably your best bet. The OS isn't your only requirement for touch. The apps and programs should be touch friendly too, and obviously Android delivers there too.
There was a concerted push a few years ago to get Android going. Apparently, that never went anywhere. This article Android 4.0 is coming! give you the official build and story. This page is the unofficial Android + Pi page: Razdroid Project Wiki though it seems they don't have much to offer in the performance department yet.
look up sailfish os https://sailpi.wordpress.com/ not sure how good it is but I believe it works with the 7 inch touch screen