The naturally aspariated 3.0 is considered bmws most reliable engine by my service advisor and by what I read.. The n55 single turbo isn't bad, but it still has cooing system failures. The v8s are terrible with lots of very expensive failure points in both the turbo and non turbo. The m57 diesels have a lot of emissions failures and problems with vacuum hoses, carbon, vibration damper etc. if cost was my only concern stick with one of the gasoline 6s. The diesel next for cost, and last is the v8s

Just my experience. I recently purchased a loaded 2008 X5 4.8i, adapative drive, multi contour seats, nav, premium sound, etc. I paid 15,000 for it in October with 89k miles. After spending 1000s in two months on maintence and repairs, frankly I wish I had bought something newer with less miles instead of spending the same amount on an older car..... basically it's gonna cost you the same to buy old and fix it, or newer. There is a reason these 60-80k cars are under 20k in less than 5 years.

I've had to replace the alternator (also belts and tensioner as preventive) ccv (smoking). I've also replaced all fluids (oil, diffs, xfer case, transmission, coolant), spark plugs, air filters, plastic rain tray, front control arms, sway bar end links, maf sensor (for performance), etc.

Plus I added the BMW hitch. That maintence, repairs and hitch cost me 4500 in parts and labor, the spark plugs, rain tray, filters, suspension, oil, I did my self. I could have bought a newer 20k car for the same amount the didn't need as much maintence.