After receiving such wonderful help from you brilliant people we've decided to get a head unit and install it ourselves (just a bit confident buggered if we'll pay $70au).. only thing left now is to go head unit shopping.
As we havn't had the factory stereo out of the car ever (Astra mark IV hatch with cd player) we don't know what size ISO cable is needed or what brands or models of head units even have them.
I'm wondering what models/brands you fellow members have in your Mark IV's, and if you needed anything else. I'm also wondering if there's multiple size ISO connectors or they're the same size, or if they come in different sizes which one does the Astra have on it?
I imagine there's going to be a bit of rewiring but we're after something mid end ($200au) that has the ISO connector so it can just be a plug and play affair instead of having to rewire every single wire.
PS: We're going HU shopping in about 4 hours, so if you don't know the exact model of HU the brand will do just so I can get a feel of who does it. No I'm not going to base the whole decision of which one to buy on the connector (that would be bonkers) but it's handy to know.
Thanks a whole bunch guys, it's really appretiated!