I just replaced an older ADT system with a new DSC system. As a side project I looked at a home automation system like HomeSeer. HA is interesting, but could be a real time sump of tinkering and complications. I gave up on HA and focused only on the alarm system. My house was already wired, so I didn't do anything with wireless equipment. Even though I didn't use wireless equipment, DSC makes a wireless version of most everything I installed.
If you don't have any system now, you need a full kit: panel, box, power supply, battery keypad, sensors, communications link. I went with an internet based monitoring solution so that I could get rid of my POTS line. It also allows me control the system remotely and notify me of non-alarm conditions (low battery, alarm arm/disarm, system faults, etc)
Programming these things can be a bit complicated because the user interface is very limited in most systems. DSC has a pc based program for programming the panel (requires an optional $25 cable) that is still a bit tricky to get used to but allows you to see all the options and is smart enough to check your settings. For example, if you setup a zone but do not add it to a partition it will warn you.
How many zones you have is up to you. I have a zone for every window, plus 2 motion sensors, smoke detectors, water detectors (eventually) - I'm around 25 zones I think. Buy a system that supports expansion for more zones than you think you need; the base cost difference is small. I have the 1864 base system. It starts with 8 zones and allows expansion up to 64. In the DSC system expansion is done through add in cards (8 zones each). I had to add a second box (and aux power board and power supply and battery) because I have so many zone expanders.
There are systems where the keypad has the control panel built in. I'd avoid those because I expect that they would offer limited expansion abilities.
You can tie alarm systems to video if your cameras have an alarm input or you have an alarm input on your video system. Then you set the alarm system to trigger the video system.
