320,000 is a lot. I bought a seven-year-old high mileage car once at 250,000 and it got me to 261,000 and change before dying on a roadtrip 150 miles from home. I wouldn't expect a ton out of it. I also only paid a grand for my 250,000 car. I wouldn't pay triple for a car with even more miles no matter how "nice" it is.
Normally when buying a car, I "pencil it out" as lasting to 200k, with maybe one or two major hiccups between 150k and 200k. More is icing, not "planned."
It'd be a gamble, and doing your dad a favor. And as others have pointed out, it's an SUV with gas mileage to match, so unless you have a specific need for that it's also sub-optimal from a running costs perspective. You're not getting a "deal" unless it's free.