Hi,
I'm just doning the same as you do (I'm also German).
1) For the map rotation. You can set the iPad to keep the orientation in Settings -> General -> Use Side Switch to: Lock rotation (German: Einstellungen -> Allgemein -> Seitenschalter: Ausrichtungssperre) and toggle with the mute-flip from the iPad!
So before every flight I just use the mute switch to lock portrait mode and release after the flight.
2) The price for the DFS-plates is here:
www.skydemon.aero/store/dfsvfraip.aspx (something around 77€ last week when I bought it)
Feel free to ask, if something is unclear.