I just did this in the fall. I installed a vista 20p with mostly wireless. I wanted smoke detectors that would trigger for the whole house (deaf son).
This is where I got everything except the wire. The main controller is hard wired.
Home Security Burglar Alarms, DIY Home Security
We now have perimeter and motion along with smoke and CO detection. There are great videos on youtube.
I can also self monitor with my phone.