Does anyone know whether it's planned to update this emulator to Android Lolipop or not?
The latest version of of Bluestacks is 0.9.24 and the built number is 5311 released on Apr. 24, 2015 and it's using Android 4.4.2. Unfortunately, there is no information related to Android Lollipop availability for Bluestacks emulator.
To download all the released versions access the XDA Wiki page: http://forum.xda-developers.com/wiki/BlueStacks_App_Player