How reliable are mustangs?
That's waaaaaay too much money for an 11 year old Mustang with that many miles. If it was a GT convertible maybe, but a base coupe? No way.
Go to Edmunds.com to price it out, but that's too much money, I don't know the options but I see anywhere from $1900 (private sale) to $2700 (dealer). There's a big hit for the high mileage.
It could be reliable, it depends on how well it's been cared for, and what has already been done to it. I would have a mechanic look it over if you can get the price down
If this is your first car, I'm guessing you are a teen, check what it will cost to insure a sporty car like this compared to a sedan, you will probably find a big difference there.