Check everything,
Air suspension Raises / lowers
Heater blows cold or hot through vents evenly front and back as they have 4 zone
No SBC warnings infact find out the date when the pump was last changed and mileage
FSH i wouldnt buy one without
2 Keys also
Check Sport settings on the dash work ok
Check gear change from cold should be smooth and i mean STONE cold.
Check if the Radiator has ever been changed 2002-2004 (04) had issues, but id ask if a replacement has been fitted even after that they are getting old
Brake hard make sure theres no Brake judder they are hard on brakes.
Equire about any exhaust work (de cat) etc as most i think have had some sort of work.
Estates "soft close works ok"
Check oil level pref with a Dip stick when hot should be around 160 on the stick.
Oil smell from vents could when warm usually indicates Rocker cover gasket leak most do at some point.
Rust around the rear dog leg of the arch if there is it could be worse behind those arch liners
Brake pipe corrosion check mot advisories
Steering lock to lock see if you hear a clonk they are heavy on bottom ball joints and top.
Vibration under load and off propshaft couplings
Check engine mounts and gearbox mounts.
Door mirros fold in and out wires can break
Damp front floor mats they can let water in around the bulk head and sunroof channels if applicable
See when air struts were last replaced usually around 90k
Water in the boot on saloons
Boosts ok under heavy multiply accelerations not holding back
Full tank hard i know who sells a car with a full tank of fuel, !! Fuel vapour smell around rear arch or in the car, lift the rear seat and have a sniff!
Ac blows cold
Whistle from the belt area idlers, tensioners can fail.
Seats if dynamic inflate and deflate when driving they bolster up when cornering if applicable
No SRS light on dash, seat occupancy sensors can fail
Keyless if applicable working on all doors and boot.
Thats all off the top of my head. Buy on condition not age id have either, the later 2004 (54) have a slighter different supercharger and 200mph clocks.