They can be a lot of fun. Back in 1988 we bought a new Cougar for the wife to drive to work. The Cougar was a Fox body, just like the Mustang of that time, but with a longer wheelbase and more weight. It had the 3.8 V6 with the automatic and power everything. I figured it to be a chick version of the T-Bird and never thought much of it until I drove it. I was really surprised how much power it had.
Ford set it up just right, it had great low-end and mid-range power. It was responsive and had very good gas mileage, about 32mpg on the Interstate. It had 230,000 miles on it when we sold it, and it was still going strong. It just wouldn't die.
It was such fun to drive that after a couple of years I spent some money on mods to get more out of it. I put on a rear anti-sway bar, a bigger front bar, Edelbrock IAS shocks, 15x7 Mustang Wheels with good, performance tires, Performance Friction Carbon Metallic brakes, and had a blast carving up the back roads. I never felt a need to do anything to the engine, transmission or rear axle, besides good maintenance.
In a Mustang that is 6 to 10 years newer it'll have more power and subtract the extra Cougar weight and it becomes even more fun.
The bottom line though, is he'll just have to drive the one he's looking at and see what it's got.