A PLC for a house alarm???
Unless you happen to be in possession of a PLC, (and want to learn how to use it for example) I would rather use a PIC or any other µ controller available on the market.
Do you have any idea what a PLC costs vs. the cost of buying a PIC? And it doesn't end there! You would most probably need to buy additional to the PLC hardware (i.e. the CPU, power supply, I/O cards etc.) the software to programme the PLC (Ladder, function block, Structured text editors etc.) and then to put the cherry on the top, most PLC suppliers today sell their products with I/O licenses as required (usually in batches of 100's or 1000's)
In South Africa, what you want to do will cost at least R 30,000.00 for everything mentioned above. (That's about $3,000.00 USD). I bet for $30 you can do the same with a PIC and some discrete components.