I just bought an 06 3.0i with 120k miles a few months back. 2k miles since I bought it. Only issue was needing a new windshield washer pump, easy fix I did myself.
Check every button, every switch, check the manual shifting, make sure you know the Bluetooth code before leaving the dealer (pair up with your phone to make sure it works), crank the stereo to make sure none of the speakers are blown and bring a music CD to test if that works.
I added a sub and BMW aux input adapter myself to make the tunes better. I found all the directions on this website (thx!).
And give it a good long test drive over as varied of road conditions as possible.
Of course, reliability long term will tell the real story, but everything I can think of works perfectly fine and am getting 19.4 mpg in mostly in-city driving overall, with a high of 25 in one highway-only run of 100 miles (straight into the wind too).
It drives fantastic, couldn't be any happier. Risky with the miles, but nowadays cars can seemingly pile up the miles a lot better than say 20 years ago - and I'd bet more on a BMW in-line 6 than just about any other engine/make. With the 3.0...wasn't nearly as worried as with a V8 given what I've read, and I was more than satisfied with the guts it does have. Not a fast truck, but not an under-powered turd either - and mpg were important to me, I'm averaging 3.5 mpg better overall than my last SUV so I'm happy there. It also tows my 2500 lb boat with ease.
In short, do your research...my first BMW so I'm betting I'm missing something I should have checked - but I think this truck is going to make me happy for quite a long time.