Q: Thinking of getting 2009 A8 with mileage above 100k. What is the most frequent problem for used A8? And what's the average cost I

Any car at 100,000 miles or more can have any number of issues come up. The things I list may not be exclusively A8 issues, but issues you can expect as more miles and time accumulate. Air suspension issues can occur - leaking air struts, failed air pump,etc. One thing I have seen a few times is the drive mechanism for the MMI screen. From the frequent raising and lowering, they can break in time. Also, you will want to be VERY careful to not spill anything on the MMI controls as they can be quite expensive - as will most replacement items. If the engine is an FSI engine (should say right on the engine when you open the hood) then the engine has a timing chain rather than a timing belt. If it is not an FSI engine, then have the timing belt inspected to see if it’s been replaced. If it has not been replaced, then have a timing belt service done immediately. Also, if service records are not available, have the coolant and brake fluids flushed as well. Oil changes are synthetic oil and with the high oil capacity - 8-9 quarts, expect oil changes to be in the $100+ range.