Is this Mustang worth it with such high mileage?

Mileage is so relative. I have driven some cars with only 60,000 miles and every suspension and steering joint is worn out. Also have driven cars with 270,000 miles and all is still tight. How the mileage affects a car is very dependent on the way the vehicle was driven and stored. Drive the car and see how it handles over bumps and how it handles and feels in general. Go with your gut feeling. Don't let the excitement of getting new car fog your judgement. This will tell you more than anyone can tell you on here. As far as safety goes there again it depends on how the car was maintained. If your not comfortable diagnosing the car bring along someone you trust and is knowledgeable about cars, preferably, mustangs.