I have a 2019 Q7. I bought brand new and had zero issues. I have the V6 with the supercharger and sometimes get 30 mpg's doing 55 mph on a two-lane road. I recommend buying Audi Care if buying a new Q7 and an extended warranty and be sure to negotiate the price of the warranty and Audi care. 34,800 miles by the way.
You should do a shootout between a used Q7 3.0t and a Durango RT for the ultimate 3 row crossover under $40k for towing a boat. Both are about equally as spacious, equally as quick, and can tow over 7000 lbs.
Might as well get a 2017 Range Rover. Anybody buying a 5 year old luxury vehicle deserves the maintenance bills that come with them. Get a certified Toyota or Honda instead.