I currently own a 2015 S4, which I've owned since 2017, and recently bought a 2017 S5 (19k mi) in February. The reason I specifically wanted a late B8.5 S5 (2015 -2017) was bc I don't like the newer B9 cars (2018- on). The newer B9 cars are bigger, more isolated, much more electronics (most I don't care about), the drive train isn't as engaging and linear, the styling is less appealing, mandatory "start- stop" and I really don't like the "tablet" style interface.....call me old school.
The 2015-2017 B8.5s have all of the latest driveline improvements available for the B8 &B8'5 cars including the improved electronics available before they ended the production in August 2016 (for the 2017 S5). I have the same options plus ADS on my S5 and the car is wonderful to drive, Both my S5 and S4 are extremely reliable and that is one of the reason I was convinced that I'd rather have a low mileage B8.5 than the newest B9. I plan on driving these along time and enjoying the sweet spot of a DSG, supercharged V6 with sport dif and all of the features I want in a DD. Additionally, my wife is a petite little thing and she could not get comfortable in the B9 nor adjust the seats to fit her. We tried the same experiments at Mercedes, BMW and all of the sport luxury cars and none meet her needs....basically she and I love the B8.5 S4 and S5 cars.
Prices have moved upward quite a bit in the past 9-12 months, so what once was a more affordable used car is $6k or $8k more than last year at this same time. A decent well cared for 2016/2017 with under 20k miles ( w/no accidents, very good condition, service records) is really hard to find. So anything btwn $36k -$40k is the going "asking prices"....sell price is probably a few thousand less from a dealer. Sounds like you may have found one of those few remaining well maintained cares with extremely low miles. If you know the history, and don't need to replace brakes, tires or other items, that is a fair price, in today's market.
The only mod I did to mine was adding a slightly stiffer 22.9mm rear swaybar to improve handling, otherwise it is a stock car.
Enjoy a wonderful car, buy it if you like it.
Bookmarks