Over the course of 17 years, we put about 250k-miles on three Mk1 TTs: 2 180s and 1 225. All quattro. (We also had two mk-IIs, but that’s irrelevant to this.)

The only consistent problem was instrument clusters. We went through 5, IIRC , all but one under warranty. No COP issues, no sludge, no t-belt issues. But, I always had the t-belts done ‘early.’

The 5-speed gear box in first/second-year 180s was weak. The 6-speed boxes were robust.

Ours were year-round DDs and eventually (a decade) the salt damage caused i$$uses for various non-body metal bits: power steering lines, a/c lines, etc. (No robust zinc plating anymore.)

The MK1s are great little cars. They are German so expect to pay to keep them running. But, outside of the occasional “problem child” where the failure bell curves of many parts overlap, they have - IMO - fewer issues that other Audis of the era.

Disclaimers: YMMV. My non-928 advise is worth (probably less than) what you pay for it. Absence makes the heart grow fonder and dims memories of traumatic events. Common side effects are: irritation and increased risk of stroke.

This was very-much certainly true for A4s with the 4-pot motors. They had - so I was told by a master tech - an oil line than ran over the exhaust side (or something hot like that) that ‘cooked’ the oil all the time.

The 1.8T engines can be fairly reliable. I put about 50k on a 2000 A4 over the course of several years. Timing belt and water pump replacement are a must if you don’t know the service history or when it was replaced (cover pops off easy for inspection) vacuum lines should be gone through and replaced as they get very brittle and will cause rough running / fuel economy gremlins. Overall, if taken care of , the engines are pretty durable. I didn’t have nearly as many problems as the other Audi owners in the forums I was in though .