Any of those years would be a good choice, there were no major changes in that time frame. Knowing that, the biggest factor is going to be your budget and what's available in your area. If you want to save a few bucks, look for a 2002 or maybe even a 2001. If you're not too worried about the money then look for the newer models.
Consumer Reports gave both the 2002 and 2004 check marks for better than average reliability. Why the 2003 didn't get one, I don't know, but it doesn't mean it isn't a good car as well. 12k would be a pretty good deal for an 02 GT. I got mine in January this year for $13,900 with 38k miles on it, and blue book was over $16k. Good Luck!
The only major mechanical changes between 02 and 04 are that the 04 cars got 21lb injectors vs. the 02's 19lb and the 04 car received a 4-wire Mass Air Flow sensor and the 02 car had a 3-wire MAF. Neither of these changes amount to more horsepower or better reliabilty. An 04 will probably have less mileage but will cost more. My advice would just be to find a Mustang between 99-04 that catches your fancy. Find one that is the color you like and that has the options you want.
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