I had a 2007 3.0 V6 Petrol with a manual box.
If you're looking for the top of the range model it's the GT you want, not the SE. I loved my petrol one, it was so quiet and so smooth. Economy on a run was up to about 35mpg if sticking to legal speeds with the cruise control on.
Suspension wise they're the same as a coil sprung C5, ball joints are the same as the hydraulic C5 and the C6. They're heavy cars at the front so are quite heavy on ball joints.
Make sure all the electrics work, make sure theres no knocks or bangs from the suspension and as it changes gear if auto. If you're going to go for one the V6's are lovely, so smooth. The 2.7 HDi does suffer from EGR valve failure the same as any modern diesel, however they will do mid-high 40's mpg wise when on a run, again, sticking to legal speeds.
They do have double glazing and the front windows are prone to delaminating. You can see it in the 2nd picture below.
Mine was a black car with black leather. I kinda wish I hadn't sold it to be honest.
If you've got any specific questions that I might be able to answer, ask away
David.