A brand new one will be around the $60k mark so I'd go for that if you want trouble-free motoring and the benefits of new car ownership (warranties, roadside assist, etc.. ) which may not otherwise be transferrable. That being said a used one with the luxury (satnav) pack and moonroof/etc might be available. The IS seems to offer good performance for what I think would be your driving skill level (P plater) and won't overwhelm you or anything.
LOL have fun in uni/college/whatever you do post-HSC because as I noted in another thread, kids who have parents that buy them luxury sedans for first car out of highschool end up being the butt of many jokes. Better off buying a car which doesn't have the luxury car badge but is just a good car (e.g. the Liberty crowd).
ps. Nice ATAR