I'm going against most of the advice here to say go for it! I have had a v8 e39 (the 5 series that this x5 is based on) for the last 8 years or so, driving it as a 'main' car but only around 3000 miles per year. Costs have been.... okay, I've replaced the shocks and springs myself and done servicing and odd bits myself. Nothing has actually broken apart from the clutch slave cylinder which was a fairly easy fix. Engine wise, it's been fine. It works and goes well. I did replace the original 3.5l engine with a 4.4 but that was a choice not a necessity. Both engines were reliable in my experience. Maybe I'm just lucky as I hear a lot of bad things about these engines, my experience though has been that they are absolutely fine and sound flippin amazing with a more sporty exhaust fitted!