I'm not the biggest fan of alarms but if the little red light puts them off then it's worth it. A Xantia looks like a Christmas tree when fully locked. The Alarm, radio and keypad LEDs all flash
I'm sure the mere fact it's a Xantia puts most off but the thought of getting past the keypad after breaking in through deadlocks must be the final straw for all but the most determined (or stupid or just plain crazy) tealeaf.
An alarm is only as good as it's visible deterrant so make sure it has some nice bright flashing LEDs on it when set.
Rather than an alarm, if the car is empty, why not just get a good steering wheel lock. They're an excellent visual deterrent and stops all but the very determined.
Else, buy an angry dog and keep it in the car.
Deliberate vandalism is the biggest problem in my mind but nothing can stop that.
Sounds like a rum old place you live in. Given that you hate it there anyway, is moving to a better district an option? Drastic perhaps but it has a lot of benefits beyond protecting the car.