The 2.7 are before a small refresh they did that changed the design of the wing mirrors and a few other bits, improved the sat nav to include a turn display in the dash. The 2.7 is not as refined, powerful or efficient as the 3.0 but not a lot different.
Servicing will cost you about 1,000 over 3 servies so 3 years or 18,000 miles (3.0D), tyres are 255/35/20s all round on the 3.0D
Xenons are a must have as the standard lights on the pre-face lift are shocking, B&W Stereo is also worth having.
MPG is obviously dependant on how its driven, but an average of mid 30's is easily doable, and on long runs I've had 45+ mpg out of my 1st XF DS and over 47 out of my current one.