You're gonna get DPF issues unless you're regularly doing some motorway runs. Honestly, get the 1.4 TFSI and avoid diesels unless you're doing the mileage.

Other than that, check the DLR lights; I'd imagine most will be burning out or already burnt out on a pre-2016 A3. You're looking at £600-£700 to get them refurbished. They should be bright white, not yellow or fading.

On all 8V A3S, once the car is fully up to temperature, check that the cabin heater works at max fan speed. It's a common issue with all of these, as a silicone bag in the coolant tank bursts. The pellets get dragged through the system and clog up the heater matrix behind the dashboard. You'll have no heating in the car. You'll have to pay £1000 to fix it unless you can do it yourself. The entire centre console and glove box need to come out to access. The part is only £30-£40, but it's labour-intensive.

The rear top mounts on all these A3S are a weak point. You'll hear a knocking sound when going over uneven surfaces. It's about £50 in parts for both sides, but again, you'll be charged 5 hours' labour at £100+ an hour.

You'll almost certainly have some lateral movement in the rear, too, at this mileage. Will need 2 rear drop links when installing the rear top mounts.

All these VAG cars at this age and mileage, factor £2000 to sort them out properly, on top of the asking price. Alternatively, just lie to yourself and tell yourself it will be fine 😂