Rent frequently from Enterprise. Based on my experience, new/low mileage vehicles appear at airports. Once they become "high" mileage, they are shipped out to local branches. Picked up a Rogue before with 55k miles at local branch.

From what I heard, the car sales division will sell the car once it is about 2 years old. They keep renting the car unless a buyer requests to purchase it (they will then pull the car out of the fleet) I do not believe there is a mileage limit. Stopped by an Enterprise car sales a while ago to check out some cars and they had a 300C with 18k miles for sale.