You have most of the hassle/labor out of the way with everything prewired. I think my controller can handle 15 zones. I know I have 11 active now.
Shop around, I'd stick with a name brand company. When I had mine installed it was through one I hadden't heard of. Price was OK but the installer never called again to see if I was happy and the monitoring company is way up in Tenn.. I was also suckered into a 5 year contract at 30.00 a month. Now that it's about to expire, and I now own all the equipment, I can look elsewhere for monitor service.