My first Mustang was a 2000 GT with automatic. The only mod I put on it was an aftermarket transmission cooler. They say the 4R70W transmissions are really stout and will last a long time if they don't heat up, hence the cooler. I thought about a shift kit but there are good and bad stories about those so I never did it. YOu can do a lot of mods to the 99-04 GT's to give you more power but if all you want is a nice Mustang with a little pep for passing cars on the way to the beach etc, an auto GT will work just fine. They are also killer at bracket racing on the strip cause no matter how I launched mine, it always came out 14.4 or 14.5 at 94-95 mph. An auto is also killer at burnout competitions lol....
One warning, once you get a V8, you'll want more power....
Enjoy,
Jordan