A family friend is considering selling an a4 cabrio with under 70k miles. I am curious about the cost of ownership. Over the years what are the common issues (small or large), such as "leaky valve cover gasket"? What are some non-obvious parts that must be changed every x,000 miles?

What should I expect if I plan to keep the car for 2-3 years?

Sorry for the very vague question, but I am making sure I can afford to keep the car in good condition. Thanks for any responses.