I would use the army of ants approach, rather than dealing with the philosophy of installers.
If you have to use monitoring service, I would suggest use two who would accept DIYers, one for $8.95 and one for $11.00 per month. One use land line and one use cell. Or a wifi phone hooked to your neighbor's router. Or one wifi phone hooked to each neighbor's router that you can reach. Hopefully if you screw up, you don't screw up all the lines.
Wired systems are more or less the same. Window/door contacts and PIRs are more or less the same. If you are not sure, just use more redundancy. Two or three PIR at an area when you can use only one will increase the reliability, and the difficult an intruder have to deal with. One PIR outside a window, one inside, a contact sensor, a vibration sensor and a audio sensor.
Wireless is a bit different, because some systems like x10 are unbelievable stupid. But you don't have to believe anybody about how secure their system is. Use element of surprise. Stick the ADT label or GE label everywhere but use x10, etc, etc. Use two systems at different RF frequency, so to disarm the intruder have to use two different set of equipments and twice the time. And why DIY systems? Nobody knows which brand and how much systems you actually have.
The usual principle is Light, Time, and Noise, to which I would add Surprise. So I wonder if all these internal alarm is necessary if you do your outdoors properly. Keep your perimeter always well lit by flood light if you don't mind the electricity bill and the environment. If you mind, maybe dimmer lights plus PIR flood lights. And then some more lights that would turn on for no particular reasons. That's the surprise.
Of course outdoor (and indoor) cameras. Wired is good but installation is a nightmare. Again use a mix of frequencies, 2.4 GHz, 1.2 GHz, and wifi (2.4G). And also 0, dummies. Use your company computer for DVR function to record your internet cams. Instead of an expensive $700 DVR, you can buy about 5 to 7 DVR's. They are so small that you can hang it up next to the cams, or put the cam and DVR inside those intimidating dummy housings. So you turn a smashing game into a treasure hunting game.
Nowadays a 20 GB DVR will last several day's continuous footage. Those with much smaller storage often comes with motion detection to save memory, and automatic overwriting old images. So there's little effort to maintain several DVR's. But whether they capture useful images is a different story. But if you don't know, who does?