Some 2015s will have the flatter gen1.5 seats, some will have more bolstered “Next Gen” seats. There were a plethora of other options at the time - air suspension, panoramic (opening) sunroof, different interior colour and trim options, hifi sound, sub-zero, 21” or 19” wheels etc etc etc. Ideally get the seller to pull the option string so you can decode everything the car left the factory with. Looking at the “additional information” screen in the software menu will also provide a lot of info. Check (or extrapolate) range at full charge, too. If you know, or are nearby, someone with the ‘Scan My Tesla’ app you can get an in-depth look at battery health, supercharging history etc.
Many of the “issues” people ascribe to the early Model S will either likely have been sorted by now (door handles) or didn’t apply to the 85D (rear drive units). If the car is being sold privately you can ask the seller to retrieve service docs etc from their account. If it’s being sold by a dealer, or has passed through Tesla’s hands in any way since (roughly) H2 2019, it will likely lose, or already have lost, its free supercharging if that’s important to you.