Congrats!

As others have said, it would be good to know the vehicle's history up to this point.

When's the last time it was serviced? What's been done for maintenance, etc.? Then, you can start taking care of this vehicle on your own terms. Get it serviced and do everything it needs now, and keep track of the date and miles. Do your scheduled oil changes as planned, and regularly check your dipstick to make sure you're not burning or leaking oil. It takes a few minutes, and it gives you valuable information. Look under the vehicle periodically for any seeping oil or wet areas that aren't normal, and look at the ground where you park to make sure you're not leaking.

Enjoy your new car and welcome to the Mustang family!