I would say to go for as new of a Mustang as you could get. The 2004 isn't bad, but if you wanted to go all out I would say go for a 2012. I've driven a V6, and it was an awesome car to drive. The seat just envelops you and you can feel the power you have at your finger tips.
My personal new favorite is the 2012 BOSS 302
Mustang. They haven't made this car for over 40 years, but they brought it back, with a 5.0L V8 DOHC with around 440 horsepower. It comes with a sexy red and black paint job as well. But it only comes in a manual.
I would suggest to go for as new as possible, and if you could get a 2006 or 2007 it would be better than a 2004, however they're all pretty good cars.