They are all very easy to install. You just have to put in the size of your wheel, add the cadence/speed sensors and that's it basically. You can buy them online from any large bike store or even from Amazon. Just check the price for the specific model that you are interested in.

Try to get a computer with ANT+ (Garmin, some other companies) or W.I.N.D. (Polar) data transmission. These are new generation digital data transmission technologies and much less sensitive to disturbances from power lines or your other electronic devices on your bike (lights, mp3 player, etc.). Such computers are slightly more expensive than simple analog computers. Still, I live in a very industrial neighborhood and benefit a lot from the disturbance-free heart rate transmission. In case you live in the countryside, they might make less of a difference for you...