If you are not into the power ... then get a GT, swap a short shifter into it, change to coilovers / stiffer springs all around, change the diff to a T2R and call it a day. You could swap cams / intake / header on the GT motor and get good gains for real cheap as well.
Or wait a year, and get the 5.0-litre mustang that is coming out.
The GT500 was sick malade fast though.