Am I the only one who thinks the OP can go a lot lower than $40K? I just had a quick look online and am seeing 2012 35i models with 31K miles go for $33K all day long. Didn't exactly have a look at all the options, but I would not focus on the fact that it has 17K miles. Your clock is still ticking when it comes to the age of the vehicle, as in, the warranty will run out whether it has 17K miles or 37K miles because you will hit the 48 month mark at some point.
Do you want to keep it beyond when the factory warranty expires? How many miles do you drive a day? A week? A month? Are you looking at purchasing an extended BMW warranty?
With a budget of $40K, you should be able to find an X5 35d (yes, diesel) with the sport package and technology package on top of what you currently want. And not in silver
Two examples:
http://www.cars.com/go/search/detail...nal&listType=1
http://www.cars.com/go/search/detail...nal&listType=1