You're right on the wiring harness and the antenna plug - like the VE Commodore, the Astra uses a Fakra antenna plug rather than the standard DIN used on most cars. To do it properly you'll also want a professionally made wiring harness that converts the Astra's radio plug either to the plug used by your head unit, or to a standard ISO connector, which you can then plug the wiring harness from your new head unit into. I replaced the head unit in my AH Astra last year, all the gear I used (dash fascia, CANBUS loom, Fakra -> DIN antenna adapter) is made by a UK based company called Connects2, they make gear for the TS Astra too, and their gear is sold in Australia by Stinger, as such you should be able to get it from anywhere that sells Stinger gear (I got mine from Autobarn)
To physically mount the HU in the dash, you'll need a dash fascia, similar sort of thing as what you'd use installing an aftermarket HU in a VY/VZ Commodore, these are also available from Connects2.
Either way, to do it properly, with the right gear, and keep your steering wheel controls (if you have them), you'll probably be looking at a fair bit of money, the CANBUS harness for the AH (which allows you to keep your steering wheel controls, as well as allowing you to set the date and time, and use the trip computer on the MFD from the steering wheel) cost me about $250, just for the wiring harness, but given that my car now has a decent stereo with full iPod integration...it was worth it.
