The two biggest factors in determining a used vehicle's price are (1) miles on the odometer follower by (2) age of the vehicle. The former has a big impact on how much 'life' the vehicle has left while the latter less so.
In plain english, the best deals on used cars come from older models which have comparitively few miles on them. They are harder to find, but thanks to online searches its getting easier. As an example, a 2012 civic with 130,000 miles on it will cost slightly more than a 2006 civic with only 75,000 miles on it, yet the older car has been used less and will last longer (because it's been driven 55,000 miles less). The one thing to look out for is excessive rust in older vehicles, which can be a problem in very snowy states but non-existent in warmer climates.
My personal preference is to search out cars that are 8 to 12 years old but with under 100,000k on them. Door dings and scratches are a mustachian's friend (it lowers the price but not the longevity), as long as the car hasn't been in a serious accident.
As for safety features, big updates in safety and crash testing occurred in '99 (dual airbags) & 2000 (rollover & stricter NHTSA) as well as the requirement of electronic stablity control ('07). There haven't been (m)any major required updates in vehicle safety since. Most of hte newer technologies for improving safety are software based, like adaptive cruise control and automatic emergency braking. Nice features to have but they aren't required and most models (even new) still don't have them. Put another way, from a safety standpoint, virtually any vehicle from 2007 is substantially more safe than vehicles from even the late 1990s.
g'luck