Mate - don't be stingy when it comes to an alarm - sure, if someone wants to steal your car they most likely would... but hey, if a good and reliable alarm; this process will sure as hell be harder and will also give you a chance to keep your car...
I recommend anything from a Viper to a mongoose. You can buy it off ebay and then go to a installer e.g. Autobarn, Strathfields, Gary's Radio and alarms or an auto-eletrician and get it hooked up.
Or, you can go to one of the above places and get the alarm supplied and fitted.
Good luck mate.