They re very reliable engines.
2009 will have the N47 engine, which if it has been regularly will be bullet proof.
This gen X3 has is a X-drive which is a 4 wheel drive system.
Do some low speed maneuvering and listen for grinding/clicking noises from the drivetrain. The AWD system is sensitive to incorrect/worn/uninflated tyres.
On acceleration any jerking/hesitation would most likely be from the AWD system slipping due to incorrect/worn/uninflated tyres or an issue with the transfer box.
My co worker has one and he averages mid 6 litres per 100km/h on long trips.
That mileage is suspect. If it is a dealership form a cbd dealership or the dealership has palasade fencing (funny but true) please be weary.
You welcome to post the link here for us to give advice, and you can send a private message to Coisman for him to check the vehicle for you.