In my brutally honest opinion, I would buy an 02-06 XC70. Its essentially the same vehicle, but on a much more manageable scale and with a more proven track record. The XC90 is a larger, heavier vehicle being supported and propelled by basically the same hardware as the XC70. All of that extra weight and volume puts unnecessary stress on moving parts. But if you've got to have an SUV...

I would only purchase model year 2005 and beyond. First year or two of any automotive model are spent working out the bugs.

I would opt for the 5 cylinder turbo engine. It has a solid, proven track record and will get better fuel economy. Avoid the twin-turbo six. More complication and expense of maintenance for not a whole bunch of extra "go." Don't know anything about very late model V8's.

Seek one with a known and good maintenance record. These vehicles are still new enough that I would expect to see complete and intact service records. Once a vehicle is neglected or abused, the next owner WILL pay for the repercussions for the rest of the life of the vehicle.