As long as you get a car fax for it, know how to get up underneath it and check for damage, and know what these cars are supposed to drive like when all sorted properly then buying an ex rental car should not be problem. Especially if it has less than 36k miles on it and still has the warranty because of that. I bought my car from a lot down here in S. Florida called Off Lease Only. They had multiple 11-13 v6 mustangs in both convertible and hardtop. They also offered free carfax with all of them on their site.
I was lucky enough to show up on a day when they had just gotten in a grabber blue rental that had the premium interior package / pony package. The car only had 4k miles on it, and was listed on the car fax as a rental. It said damage rt rear. The body work was perfect, looked show room new. Got up underneath it and the unibody was fine. The end of the story is that I got a 13 v6 premium with black leather, blue tooth, my sync and ect. for 18,500 no negotiation. The car stickered for 10k more than that and still smelled new on the inside.
Have had zero problems with it. These modern cars are over engineered. So even if every a hole that drives it stomps the pedal to the floor over and over, all that does it break it in properly.
2013 Grabber Blue Premium Couple, Auto, FRPP N Springs, FRPP Dampeners, Lund Tune