Just bought an 85 300SD a few months back, it sat for 17 years until I bought it. Drove it 5 hours home that day.
I've done the basics so far:
oil change
brake lines flushed (lines look good)
alda and egr delete
fresh air cleaner filter
fresh tires
compression checked out
The suspension in the rear is almost completely shot.
The odometer broke at 200k miles... in 2001

For what it is and how long it sat it runs great. Shifts great. Just looking for some more longevity tips really.

Oh yeah, and we put on a side exit... for fun.

 

check all rubber suspension parts and ball joints etc.

Wheel bearings etc
Prop shaft

 

I just noticed some huge exhaust dump pipe just behind front wheel - OMG