Don't get stuck with a cheap or expensive system that doesn't do what you want.

Consideration #1:

Generally people want a camera system that records only when something is happening (ie. Motion).

If you record continuously, you will not be able to effectively review your history because it is to large (unless you know what time an incident happened).

Consideration #2:

The system should be accessable via the internet for monitoring and reviewing recordings.

Most systems today can be accessed over the internet, many don't allow you to review recordings remotely. For instance you are out of town and you want to see what was recorded yesterday.

Consideration #3:

FPS resolution (frames per second) is important if you want detail for prosecution or license numbers.  This involves the DVR and camera combination.