I have a 4.8i and I love it. It is however very thirsty. I think that for the vast majority of people the 3.0i is more than enough. I test drove the 35d and I was impressed by it. I think that if I would change my 4.8i I would buy a 35d.
I've had two X5s and I had no problems except for bad O2 sensors, a failed ignition coil and a burnt bulb. Other people have had nightmares in terms of reliability. Take in mind that the X5 are usually severely abused by their owners with the resulting stress on the chassis and its components.
A 2008 with 38K miles seems to me a bit high in mileages. Personally, I've had two X5s in the last 44 months and I have accumulated only 31K km (19K miles) but I am the exception rather than the rule.
Other than that, I highly recommend the X5.
Good luck.