I'd highly recommend the 2015 Mustang IRS, which comes with either 3.31 or 3.73 gears. Go for 3.31 if you want it to be more tame and efficient or 3.73 if you want it to accelerate faster.
I personally opted for a Levy 347 stroker (rated at ~450hp) a T-56 Magnum 6 speed (which will require a fair amount of work to fit but offers same ratios as the tko 5 speeds which fit right in) and the 3.31 rear end ratio.
This combo should be plenty manageable and driveable but also will be quite good at the autocross and track, kind of a pro-touring setup. Kind of like Johnny Depp, can play any part very well.
Don't go above 500 or 550hp or you'll hit Arnold Schwarzenegger status, exclusively super intense and powerful. (AUGH)
Or you could opt for a 300-350hp setup which will be pretty mellow but with a very healthy amount of get up and go. (Oprah maybe?)