I will argue that drive units aren't really that big of a deal. While there is some aftermarket demand for drive units, it is fairly small and there is no shortage of S's being torn down every day with drive units being removed. On top of that some of the most common failures can be serviced by a motivated owner. Also so long as the car has a recent enough iteration of the drive unit I wouldn't be as worried about it.
Batteries are an actual challenge, but if you work with someone to upgrade an 85 to a 100KWH pack, then you get the faster Supercharging speed and possibly better performance, in many ways making the car better than a somewhat newer 90 version. Just be aware that there is plenty of OTHER stuff that breaks on these things. Front end work like ball joints are frequent issues, and the rear drive shaft spline regreasing or replacement. Then there are the usual problems, door handles, screens, roof (if you have that), air suspension leaks (if you have that), brakes pads seizing in the calipers. So if you have the cash to not only buy one, but maintain it over the long haul then yes, I can see buying a late 13 or 14 that is completely out of warranty. There will almost certainly come a time when I do exactly that! I could even see shipping it to my favorite aftermarket guy and having him upgrade the pack straight away to a 100KWH.
If you can only afford a 25K car and that is it, then this is just like any other 25K luxury car, probably too expensive to maintain over the long haul. Not saying you can't get lucky on these though. But I wouldn't buy a 25K Mercedes S class and think it will be cheap to live with. These are probably better than an S class, but still, this isn't a Toyota Camry or Avalon.
The current car market is a bit crazy right now. Earlier this year I saw quite a few S's for under your price point. You would probably be wise to wait until next winter when the market generally calms down again and shop then.
