Make sure all electrical items work and ac.
Has the gearbox been serviced?
Has the quattro system been serviced?
Are the tyres the same brand and tread on each axle as uneven tyre wear or mismatched tyres can damage the quattro system.
I'd also recommend you check out buyers guides on YouTube as they'll help.
high mileage cars can be reliable if looked after. My A6 is on 344k miles, has been owned by my family from new and is over services if anything. I'm a stickler for maintenance though.