2.5 petrol engine will be slow and will need revved hard to get it moving, 3.0 petrol engine has the grunt so will seem less athsmatic. Petrol engines have timing chains, though look out for perished breather pipes causing poor running and EML lights on. Check all 6 spark plugs have been changed, one bank requires inlet manifold removed to access plugs. Misfires could be down to dead coil packs, each plug has its own coil pack.

2004 my onwards refresh are the nicest looking and alot of the suspensiom/drive train made it into early XF's

2.7d is very torquey good power, though euro 4 emissions (straight exit exhausts rather than exhausts pointing down) can suffer from perished deactivation ports which stop the DPF from regening. Flexi pipes on catalytic convertors can split over time which give smells of exhaust fumes in cabin.

Wiring behind front bumper can corrode causing issues with the climate control system and can cause the alarm to chrip at random times.

ZF auto boxes are fairly robust, however manual gearbox can be fairly agriculutral in how it feels when changing gears.

Check for signs of leaking rear diff output seals too.

Apart from that they are cracking cars to drive and look at especially with the right kit!

Hope this helps and sorry if some of my spelling is dodgy!

Oh and check the sills towards the front, corrosion can set in behind the plastic covers. Look up from the underside were the jack points are exposed.