Yes I realise. My bet would be single skin.
A demo saw will make quick work creating an opening. You then only need to square set the opening and do some carpentry work on the lounge room side to fit a frame and sliding door. You need to put a frame that's larger than the opening so that you...
Here is a link for a contractor in Sydney who does the suspended ceiling in units.
http://www.kkplastering.com.au/Index.asp?pagename=Vermiculite+Ceilings&site=1&siteid=330
He has a youtube there of the process which you may find interesting.
I think he was quoting about the $3.5k mark...
