Both of my Model Ss were pre-face-lift. First one, a 2012, sold at 82,000 miles, nothing wrong with drive unit that I know of. My "new" Model S, a 2015, has 93,000 miles, drives just fine every day. I would guess that drive unit replacements are not the norm, so why are you worrying? Any idea how long the drive units are warrantied for?