Some time ago I was interested to perform HUD retrofit. The informations I've got from a professional installers were that the original factory fitted dashboard can be cut without any problems an even if the cut is not the cleanest one the trim you should buy is covering perfectly any imperfection.
This is the trim link
http://www.realoem.com/bmw/enUS/part...&q=51459239336
As well you should consider that most probably your cars' instrument cluster is not appropriate for retrofitting the HUD, check what you have and compare with what is needed (use realoem.com)
The last but already solved by you is the windshield
Whish you easy retrofitting