I've installed heaps of alarms. They are easily bypassed. Nobody listens to sirens. Immobilisers are only as good as the guy installing them. Good for central locking and auto window up. Good for insurance.
A very old school trick I picked up was to run a small switch into the loom to earth the coils packs, the car might wind over but never spark, but the time they work it out the battery is flat.
Another thing, there are a lot of unsavoury characters who will just want to pop the bonnet to grab your engine goodies. I have relocated my bonnet latch and installed a dummy one.
Where in Melbourne are you Oscar? I have a couple of mates who are auto elec wizards and could probably do the job for you. PM me if you like and I can see if they are able to help.