1. MPG w/3.0 - searching reveals 15/16 city 20/22 highway - valid?
I have a 4.8, see below.

2. 3rd row seat - seems rare in my searches. standard on a GL
It's really a hit or miss on the X5, some have it some don't. I personally don't because the backseat is way more comfortable when you are not equipped with the third row.

3. Sport pack on a 3.0 - seems pretty rare. I wasnt SP for the seats and the sport steering wheel
Again, it's a hit or miss.

4. Can i swap the steering wheel out for an M wheel? I love my E90 M3 steering wheel.
I believe there is an M-Sport pack that adds an M wheel so I dont see why not.

5. DVD - this seems very rare, sucks. I have 2 kids under 2 so this is a big deal. Any cheap aftermarket, tasteful options?
DVD is pretty rare however there isn't any X5's that don't have the DVD system. Again, it's hit or miss there is so many ways to equip an X5 it will be hard to fine exactly what you want right off the bat.

6. Of course I prefer the 4.8. AND, they seem to come better equipped than the 3.0's. What MPG can I expect with a 4.8? I dont mind spending a couple extra $k on a 4.8 loaded up. But the MPG is the killer here.
I can average about 18.2, 18.6 if my foot is being really light, with mixed driving, however when I drive in the city it quickly drops to about 14.5.

7. My search has found extended warranty through fidelity as a good option for around $2k for 3 years. Valid estimate?

8. Extended maint @ $1800 - since the main expense is brakes, I dont think this is worth it. I plan to buy an X with 25k miles or less and only drive about 12k per year so probably wont get my money's worth...
Keep in mind oil changes are also included and wiper blades and pretty much everything on the car. I would get the extended maint.

Hope that helps, you can go to a dealer and tell them exactly what you want and they will be able to get it for you by doing a dealer trade and then you get the car you want, down to the DVD system and 3rd row seats.
You really cannot go wrong either way you equip this car. I have a 2008 4.8 non sport package and the car is perfect I wouldn't change a thing about it.
Another thing I would recommend getting a car with is Comfort Access, a feature that totally spoils you. I have gotten into other cars with conventional keys put my foot on the brake and tried to press a start button that didn't exist. I also have the comfort seats which are VERY comfortable and very supportive on long trips you do not fatigue as fast as other cars you are much more supported.
If you have anymore question ask away.

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2008 BMW X5 4.8i, Silver, Black Leather, Custom Flowmaster mufflers with Hushpower, Comfort Access, Cold Weather pack. Premium Pack....