Best guy in Sydney (IMO) is FHRX Studios in Caringbah.

He aint cheap but he does a premium job that looks factory fresh. Generally he's booked out about 3-4 months in advance.

Best you talk to him about your options but at a guess your package would involve dynamat type material in the doors, sealing & beefing up the existing speaker mounts, possibly some upgraded wiring. If you wanted more base then you might be looking at upping your budget a fraction for a sub & amp.