If you are looking at a used Mustang you may want to expand your search back a couple of years, as there are many of these cars that people do not use as daily drivers, that are garage kept, and are far all practical purposes like new with low mileage. For all practical purposes the 01+ GT's are identical, the biggest mechanical difference that comes to mind is the change from seperate MAF, and IAT sensors on the engine to an integrated one in 02 or 03, and a slight change in interior around the same time (it seems that they moved the location of the defrost/fog light switches about 2 inches), there are other very minor changes each year for example newer 04 mocels don't have cut off switches on the center A/C vents. As to the 99 and 2000 models, at first glance they appear almost identical to 01 and up model, however they should be noted as seperate from the 01 and newer GT's for a variety of what may seem nitpicking reasons, the engines are slightly different in a number of ways as the 99 &00 engines were built in the Windsor engine plant, and 01 + built in the Romeo plant, the exterior does not have the raised fake hood scoop, and has smaller side scoops, and the interior center console is of a difffer (poorer) design (bad location for cup holders, etc.), also the 99&00 use two single DIN slots for the stereo/CD player where the 01+ uses a single dual DIN slot (an issue if you will be upgrading the stereo).
hope this helps Ike