I think the most important thing (and one that ard mentioned), is buy the youngest car that you can afford.

My 2008 X5 has been great; it's 5-years old now. If I had to replace it, would I buy a 2008; hell no; I would buy the newest (and most powerful) model that I could afford...

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2008 X5 4.8i | Wheels: Summer> style 336 20" RFT; Winter> style 214 Ferric Grey 20" RFT | Aero kit | ESS E-Flash tune | V1 | LCI Rear Tail Lights | Coding completed | Engine replaced at 90K