If you are trying to predict a car's reliability and future value you might as well take the money and gamble in Vegas--there's no way you can predict that. As far as the car itself it is high mileage, 2 owners, the tires look shot (as you mentioned they are giving you some money for that). It you are only looking to keep it until Feb '13 why just not just get a used Accord, etc ? Not sure why you have to buy an X5 for less than a year.

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2008 X5 4.8
Jet Black/Black Nevada/Dark Burl Walnut
Adaptive Drive
Premium Pack
Tech Pack
Climate Pack
Sport Pack
20" Wheel Option
Premium Sound
Sirius Satellite
Rear DVD Entertainment
Multi-Contour Seats
Comfort Access
3rd Row Seat
Heated Front Seats
Running Boards
3M VentureShield Paint Protection Film