Just done an oil change myself. Yep its 8 litres (Mobil1) and it couldn't be easier to do yourself. That said my xfire is out of warranty now so no worries.
Spark Plugs shouldn't need changing till at least 70k and the NGK Iridiums are pretty expensive probably around £9 per plug I believe and you need twelve
My 1st service (12k) was quoted at £550 by DCUK but as you don't have to be tied in to dealer service for warranty purposes I used a friend in the business who was an ex Rolls Royce mechanic and he did the lot for £250.

My advice is shop around don't pay dealer prices.