i'm in country VIC, otherwise i'd try and check the cars out for you.
an '04 would need to have the belt done regardless, because it's over 3 years old. if it's not done, then it needs to be. so you would push for an instant $1500+ discount! but you then should get it done before driving back to NSW, or risk a blown engine and $4K+ down the gurgler!
if the car has a FSH through an alfa dealer, then it should have been done already. you should probably check before making the trek down.
in SYD, one of the best indy mechanics is Max Oddi, that a lot of SYD alfisti use. he is very familiar with the GT, and has fitted the Q2 to many of them.
actual tuners are scarce. not a big market for it unfortunately. most of us in Oz end up buying parts from overseas - like Germany, Italy, and the UK - and have them fitted here. so it's a matter of picking the parts you want. but there's plenty of good info and opinions about such things, including suspension, exhausts, brake pads, etc.
but in all honesty, i wouldn't aim to make the GT as quick as an STi. it's not that kind of car, and it won't be easy to do. except with a supercharger, but that's not only expensive, it will obviously put strain on the whole drivetrain, meaning potential breakages and upgrading of clutch, etc. though one of the SYD guys has a an Autodelta SC on his 147GTA which is apparently awesome...except he keeps breaking things.
note, if you want an excellent GT local to you, with top quality tuning parts already fitted (KW coilovers, Q2 diff, Mille Miglia catback exhaust, OVER Racing titanium strut brace, GPS, etc), then see Mr Huxtable. i presume he is still wanting to sell, and hasn't already sold. the online ads have been pulled, but i suspect it's cos they expired. he was asking $39K last i heard, but that is fully loaded, and probably the best (or secondbest
) GT in australia: