Since Android 4.2 there should be support vor Miracast. Also the Samsung Galaxy S2 Plus has Wifi Direct. But I can't find any option.
Does Samsung Galasy S2 Plus supports Miracast? If yes, where can I find the option in the menu?
No, I don't believe the SGS2 Plus supports Miracast. This feature comparison shows the other phone having it but not the S2.
However, from my understanding, since Wi-Fi Direct is supported it shouldn't be extremely hard for someone to get Miracast working on the device. This has been done for the regular SGS2, though I do not completely understand those posts.
Yes it does, but probably not on the official ROMs. It is working on CM12 and recent CM11-based ROMs. The broadcom chip inside galaxy s2 plus supports multi-role (but with probably newer drivers), so it even doesn't disconnect from wifi while it is mirroring.