also, looking at it he has had it for a number of months going by the outside temp displayed on dash.
Look through all MOT milage and verify it is all genuine. Brakes, check these, run your finger across the disk to see if it lipped at the top of the disks. New disks and pads will set you back a couple of hundred quid so getting something with half decent brakes will last a couple of years. These are heavy motors.
To be honest, big cars do tend to sit about at dealerships, people scared of their running costs. So don't let it put you off. If it doesnt look right in any way then walk away. A well maintained GS will look after you as long as you look after it as well.
Be aware that these are very nice cars, they will give you a nice ride and 1st class ride and all that, but expect to spend a little on fuel, they are not massively excessive but the hybrid is a better option.
LPG install from Profess Autogas is a good option for the future.