I own a '14. Bought it used in late '18 with about 60k on the clock. Currently sitting ~125k and out of AEM warranty. It's my only ride.
It will not compete in FE with any other TDIs, but for the pig that it is, and for how much it can pull and how capable it is in overall versatility, you can't touch it. I do my own wrenching and maintaining on it, outside in the elements, with basic tools and VCDS. Due to complexity, I wouldn't entertain even considering buying one without being able to scan it, and knowing what to look for\at.
As per above, parts & even regular maintenance stuff is more costly vs some 20 year old ALH, etc. Keep in mind that you are talking about a top of the line offering vs the other end of the spectrum, so costs and availability of certain parts can become an issue. If you rely on a dealer for work\maintenance, each visit will run you min 1k regardless of what it is.
There were no '17 TDIs. I'd suggest staying away from the '15+ as they have lots of issues with dying eyelash in the headlights, which requires a very expensive new headlight. There's also tons of failures with ACC, so steer clear of that option unless you have money to burn.
During my ownership, I've only done the following:
- pads & rotors all the way around once (will do again this summer)
- replaced one ECT sensor (PITA)
- replaced one set of tires (old ones were originals)
- changed diffs and transfer case fluids
- replaced aux belt and all pulleys\idlers
- replaced battery
- lots of oil\filter\fuel filter changes
Current issue - downpipe flex is leaking, so I'm driving around with a MIL. Have to find time to take DP off to replace the flex section as it also contains the primary cat, so a new replacement is a few $K from VW.