Customers who always look for the cheapest are a nightmare imo, sell your service and don't be afraid to charge a fair price for it. I would have more respect for someone charging me a good price for a good job.
Hi, Yes if you connected something external to a zone it would activate on set.
I personally would look to install a stand alone system, something from here maybe.
I've had them for years, just bought a new one. I've been lucky enough not to need it in anger, but I am aware a large percentage of the time I'd be crawling through the wreckage to eat the sd card.