you dont have to fear the w140 breaking down, you fear its express windows, air conditioning, headlight wiper, or close assist breaking down.

I've had a 97 s320 for a year and a half now
I replaced the evaporator core which took a month (a slightly laid back schedule =) )
Power antennae won't roll all the way down
Power window switches broke and collapsed through console
Clunk in the rear end
Headlight wiper sprayers stopped up
Headlight lenses cracked (rocky roads around here)
Windshield wiper axial extension function broken (still wipes fine though)

It's really a fine car though, nothing better for vacation or extended drives.

Just make sure you dont have to replace an evaporator because it'll cost you 2 or 3k and if you do it yourself you'll commit suicide.