I've been with alarmforce for many years and no issues *knock on wood*. When the tech comes in, you would just have to tell them that you want sensors in the garage as well as any other areas you want monitored.
If you want to go high-end and fully custom, contact
http://secureye.ca/ and ask for Eugene. We use them for our office security, IT, and our company vehicle tracking. If you can imagine it, they can do it.