I can't say much about Mustangs themselves, but as a person who's been using older cars for a long time, I'd say it strongly depends on a previous owner.

If the car was serviced regularly and in general taken care of - it'll be a great deal. If not - even the most reliable car in the world can be put in a very sad state.

If it's in great shape but has been sitting for a while (a year or more) - expect replacing several rubber gaskets/hoses/bushings/etc. because they tend to solidify, dry out and crack if the car isn't regularly driven. And replacing the liquids like oil, brake fluid, coolant is also a good idea in this case.

The fact that most people theese days tend to think that a car turns into a pumpkin (like Cinderella's carriage) after 100000+miles just means that those folks don't know much about cars.

And yes, older engines are less efficient, but on the other hand - much more durable than modern ones.