I've owned an '80 300SD since new, fine car, comfortable to ride in,
great visibility and simple enough for a DIY owner to maintain and
Rust and the climate control are the '78 - '80 116's problems. Rust is
obvious, either it's rusty or not. The climate control is very
complicated and expensive to repair so as these cars lose value some
owners try DIY repairs which only make things worse. So if the climate
control is busted don't let someone tell you "Oh a new fuse will fix
that." The cost to make it work could be as much as you're about to pay
for the whole car!
The engine makes 110 HP and is good for 250K miles if reasonably
maintained. It should have its oil changed every 5K miles and its valves
adjusted every 15K miles. A diesel's internal condition can be checked
by a compression test, a worn engine will be difficult to start in cold
weather so have the compression checked if that's a concern.
Most of the cars that you'll see are nearly over the hill - shabby
interiors and lots of miles - be patient if you want a good quality car,
they DO appear from time to time and if that's what you want go for it.
I wouldn't ignore the '81 to '85 300SD, if only to avoid the older car's
complex climate control, otherwise same size and drive train but more