Sorry to hear about your old Mustang.
Mufflers look like GT500s (tips are not rolled).
Raxiom taillights (American Muscle sells these).
Headlights look like a set offered by American Muscle.
Wheels do not look stock to me but I could be wrong.
Grille is aftermarket.
Take a photo or two of the underside. Maybe it has some suspension mods (LCA's, sway bars, etc.). Maybe an x-pipe is lurking under there?
Any maintenance records? If a vehicle has mods but decent maintenance records then I would be less concerned that that horse was ridden hard and put away wet.
Another thing is that some people cannot help but modify their Mustang. The goal is to make it a little better than when it left the factory, to get some enjoyable wrench time in, and make the car more unique. It's a great hobby (addiction!) and plenty of modder's cars never see the track. So, don't fear a modded car, especially if you can get your hands on the previous owner's paperwork for the mods.
Just my $0.02.
2012 GT 6MT - RTR #59; performance white / charcoal interior
3.73's, AED tune, MGW Race Spec shifter, Airaid MFT, BMR rear LCAs & relo brackets, Panhard rod & brace, FRPP swaybars, MRT a/c H-pipe, LP OTAs, FR GT500 Touring axlebacks, FRPP diff cover, JLT PCV p/s catchcan