Low mileage does not mean "1/4 mile at a time", in all cases, lol. Some people don't drive much or they keep the Mustang for nice day driving only (or both). And summer (high perf.) tires do tend to wear much sooner than the usual all-weather tires. Having said that, some Boss owners take their cars to the track to run them for what the car was designed to do. What's missing at the moment is information.
The clatter under the hood could be problematic. Maybe the sound-deadening blanket has been removed. It would be worth asking for the maintenance records and the OASIS (Ford's service system) history. If no history is available then that could be a troubling factor but it shouldn't be a deal killer. The salesman might put an genuinely interested buyer in touch with the owner to get the real story. If the engine needs work then the buyer can negotiate the price down to account for the needed engine work.
The Boss would be a nice project for the right buyer. If that buyer is you then go back and see what info is available for the the car and see if you can speak with the owner. Then, if all is good, haggle with the salesman to get a fair price and then you have a solid car that needs a little TLC and elbow grease to make it a great car again.
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