The big thing to research is the lens used on whatever you buy. Make sure it will see the distances you want to see without the use of digital zoom (digital zoom is a joke). Many folk will get a small cheap camera, mount it 20' up on the side of a building and want to see a sidewalk or path 30' away, then complain about a poor camera; when in reality the small cheap cameras are meant to be used at a range of about 15' or 20' (with an ideal range of 6'). They aren't much different from cameras that are meant to see 50' to 75', the main difference is the optics used in the lens. The price increases of better lenses and high end lens controls gets rather dramatic.
Second thing to research is the final resolution. Making sure you have the proper optics to frame and focus at the range you want to see is primary, but having a good final resolution is also key to identifying a badguy, or just having a general description such as the color of clothes the badguy had on. Old analog systems started out at 480x240 and go up to about 800x600. Digital systems start at about 640x480, and they go up to full HD resolution of 1920x1080. It won't be long before digitals start showing up in the 4k HD range (4096x2160). Higher is better, and slightly more expensive. The key thing that increases expense on higher resolution cameras is video storage. If you have a camera that captures thirty 1920x1080 images every second, it requires a vast amount of storage to record and have much recording time. A couple HD IP cameras can eat a large hard drive up in a day or two... Recording only when motion is detected is a good way to not waste space.
Researching these two items should narrow down the field of available units significantly. I personally like Vivotek brand cameras. They aren't the cheapest nor the most widely available, but they have a powerful software package included and have a great selection of features and models to choose from. They can record to internal storage (similar to a game cam), network storage, or a dedicated computer system. There are many ways of connecting them also, many times there are existing cables that can be used, even power wires. Setup also has a bit higher learning curve than most folk prefer, it helps greatly to fully understand IP networking. I can post some sample shots from some of the Vivotek installations I've performed if you are interested in their lineup.
:disclaimer: I am in no way affiliated with Vivotek other than the fact I have shopped around to find what would serve my clients best, and they were what I came up with.
