1.8T's can suffer from oil sludge in the engine. This is (arguably) resultant from Audi speccing incorrect oil and intervals at model launch. Audi issued a Technical Service Bulletin some years ago and there was a class action lawsuit in the US.
If you have enough sludge, the oil pick up tube in the oil pan can become clogged (amongst other things), so starving your engine of oil, and so posting a warning on your DIS -the light you have heard of. If you see the light, you have mere seconds to pull over and engine off before you fully toast the engine. From there it's a decent job both to replace the oil pickup tube and another story entirely to rid the engine of sludge.
So, you want to look for a car that has spent at least from 60/70k miles onward of its life on fixed interval service -every 9-10k miles or better and NOT continued on variable service (20k miles). This will show you sludge would have had less of a chance to establish. Regular service intervals go without saying. If you see those two things you are largely de-risked. You also want to see fully synthetic oil changes. If you can see a receipt for the oil pickup being changed at some point in the latter life of the car then great -though this may suggest problems in itself (that the car ever had a problem with sludge).
The only real way to check properly for sludge is valve cover off (which is simple though will require a new gasket -1.5 hour job) or drop the oil pan (which is hard) or install a oil pressure gauge (simple enough). As I say, a decent service history should put you at ease. See my recent posts if you'd like to know more.
More info:
https://www.audizine.com/forum/show...essure-Survival-Guide-and-troubleshooting-DIY
OTHER THINGS
Rust on front wings caused by wing liner rubbing over time. Front wings can be swapped (they're not welded).
Fuel line rubbing on welding crease near the rear offside wheel. You'll smell it if/when it goes. Check it's not worn, or it's fuel tank out to replace
Don't know if the cabs get this but rear number plate can rattle eroding paint causing rust -check that's been prevented
Remove oil cap with engine running check it doesn't idle rough or you have suction holding down the cap (PCV system compromised. Most parts are cheap though)
I'm on 160k original clutch original turbo
Look out for coil packs -Audi recall. The originals were bad (likely would have gone bad by now on any car you're looking at)
Valve cover oil leak or half moon gasket on the cam chain tensioner -very top of engine, all around and especially check the rear by forewall. £30, 1.5 hour job
Scan it through the ODBII port with VAG-COM if you can
