Do all electrical items work? AC compressors can be expensive to replace if it's not working. Also if it's like the A6 of the era I wouldn't be surprised if it's hard on suspension arms. I'm assuming it's full service history?
If xenon headlights make sure these are working as theyre expensive to change/fix
Why is the owner selling it?
I'd recommend a diagnostic check to see if any fault codes flag up. Sounds like an issue with central locking. You might find it's the drivers door module that's failing. As this module controls the rest of the central locking.
If bought within 30 days you have the right to get the seller to fix it.
Could something be blocking the roof from closing/opening. We've had one case where a shoe was blocking the roof from opening.
What's the service history, does the roof work as it should, are the tyres the same brand/tread depth on the axles as uneven tyre wear can damage the quattro system. If xenon headlights are they working? Does the AC work?
The CVT gearbox is known as a weak gearbox. Has the car been used for towing? If so that won't have helped it's condition. I think the modules are known to be more unreliable on the CVT.
My advice would be to try one out at a car supermarket like available car.
The A5 is a low car, which should making it easier to get in and out.
I think as long as you're not expecting to have any passengers behind the driver's seat then there should be enough room. Also as it's s line trim the seats will have more side bolsters as they're sports seats.
Is there a reason why you can't try the car you're being offered?
I think a module change will be a few hundred quid. Obviously worse case scenario is a knackard gearbox. But at this stage my money's on the module or sensors.
If you've bought it from a dealer in the last 30 days then they have an obligation to fix it. Or you can ask for your money back.
If it was a private seller then they might have fobbed you off. Usually a gearbox oil change is the first thing to be done if the auto gearbox starts playing up.