I've very recently bought one of these of the same year. I can answer some but probably not all of your questions to some degree.
1. 2017 if it is the FL model is belt driven. Mine has done 66k. I phoned my local Audi dealership and they said it won't need to be done. They changed the service interval from a set mileage and 5 years (whatever comes first) to just a mileage of, and don't quote me on this as it's from memory, around 122k miles. Decision whether to change according to their previous advice (lower mileage or 5 years whatever comes first) is up to your own discretion. Belts have improved no end nowadays. Personally, I'll leave mine until about 100k miles and change then.
2. Thankfully no problems with mine. Just the usual service items needing doing like filters and plugs.
3. I'd look for as much Audi or independent specialist service history as possible. Ask the seller for a print out of the Audi service history or receipts from independents or previous owners. I'd also take your time choosing one. There's plenty out there.
4. I like to do preventative maintenance myself. The pollen and air filter are very easy to do. Finished those. I expect the fuel filter is an unscrew old and fit new easy job too. Spark plugs are a different issue. It's tight under the engine bay despite there being bigger engine variants of the same body type. Basically, I am changing the core renewables anyway just to reset the clock on them and then will replace in the future according to my fitment of them now.
5. Fuel economy looks good thus far. I am not one for noting mpg figures but it appears very economical to me over the few hundred miles I've driven it thus far.
6. I have both DAB and USB. Mine is a S-line model but has very few options, so judging by this only, on this trim I think it would be standard. Unsure about the SE and Sport trims though.
Can't really be much help other than that, I'm afraid. Someone more knowledgeable than me that has had their car longer may be able to add advice or give better.
Best of luck with the car search if you haven't found one already!