Build your own home security system?

What got me thinking about this was how cheap all the little sensors are. You can get sensors that will send a signal when a door or window opens, or when glass breaks, for dirt cheap. They're so cheap, that they're almost not even noteworthy when it comes to cost. However, if you were to attempt to put together your own home security system, how hard would it be? Being a standard "computer guy" my first thought is that you'd have a program running 24/7 on a highly efficient, power sipping system. Something like a raspberry pi, although that particular system might not be ideal, I have no idea at this point.

So you'd have all your sensors, that were cheap. But there are still a few hurdles to get over. At least for someone like me anyway, who has next to know experience with building security systems. So I'll just number them.

1. How would you get your sensors to wirelessly talk to the computer program which was monitoring them? No one really wants to run 50 new wires through their attic, once for each window and door. Do some of the sensors actually have wireless capability? The cheaper ones didn't really mention anything about it.

2. Where would you get the computer program? I'm assuming that somewhere out there, there is a program that can be purchased that will tell you when something trips a sensor. But it also has to sound a loud, audible alarm, and call the police and play back an automated message. This is what those other security companies do.

3. You'd need a keypad for disabling it, at your front and back doors. How would that tie in? I'm assuming this keypad would work with the computer program that's running the show. Any thoughts on this?

And other than that... well I think that's all there is. Would you consider this an adequate home security system for a typical suburban home? Its got sensors that tell you if any window or door is opened, and glass break sensors on the windows. You trip one, and the loud alarm goes off in the house, and the computer program calls the police. And of course, when they pick up, the hear an automated message just like they do with ADT and the rest. I know that ADT has live monitoring, but I can live without that. To me, that's not such a big deal. The system also has keypads at the entryway doors, which allow the homeowners to enter a code and disarm the system, or rearm it, depending on what they want to do at the time. This system is simple, of course, but often times simple is all you really need. Thoughts? Opinions?