Word of advice. Look for an Xbox that has a 175watt power supply.
This indicates that it has a new "Falcon" CPU and motherboard. These run significantly cooler than the older ones. This will decrease your chances of having a Red Ring of Death error.
Also, they usually have a BenQ DVD drive which is less prone to breaking than the older models.
Here's a link to find out how to tell if it has a 175 watt power supply.