There is no official mileage where mustang engines start going bad. I've had a few mechanics tell me that generally 70-100k is when things are going to start breaking if they are going to break, but it all comes down to maintenance. If that car had been rigorously maintained, a 200k engine can be just as health as a poorly maintained 50k engine.
If you buy from a private seller, ask some questions he or she would have to think about to get the truth out. If you ask about oil changes they are going to obviously say "Every 3000 miles like clockwork!" If you want the truth, ask them if they'd ever changed their transmission fluid or performed any part replacements. Just things that people don't automatically have a "Safe" answer for. Like the Transmission fluid one, some people say change every 45000, others say never change it, ever. So it's a trick type question.
2004 Sonic Blue V6 Automatic ~ Daily Driver
(Standard Bolt-On's) 20 MPG
2003 Torch Red GT Convertible Manual ~ Toy
(Looks good, Sounds Good) 20k miles
You make Lemonade, I'll throw the lemons back at your face.