Hi Russell. You will see quite a variety in mileages, as many estates are used as family workhorses so cover plenty of miles every year. Others are chosen because they are more suitable (dog owners, for example), but may not cover many miles.
15,000 miles on a three-year-old car shouldn’t be anything to worry about, and would normally mean it’s a bit more expensive than a similar 30,000-mile car.
Check the service history and make sure that all services have been done on time, every time – even if the mileage is low.
If the car is less than three years old, it should still have some new car warranty remaining. If you buy from a franchised dealership, it will probably come with a 12-month manufacturer-branded warranty. If you buy privately and the car is out of its new car warranty, you have no protection unless you choose to purchase an aftermarket used car warranty.