I still recommend you buy new, albeit the diesel's residual is slightly lower than its gasoline counterparts, the X5's relatively strong resell value should convince you to shoot for new. For me it's a no-brainer especially since you're buying so close to new, savings of used vs new is negligible.

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2009 GT-R super silver / 2010 CTS-V raven black / 2011 X5M alpine white