The diesel coupes are popular with some and tend to be a little more valuable, all things being constant, than the 4-door 300's. Mechanically, the two cars are virtually identical so its all in the look you prefer. I'd say $2500 with 120 k miles is very good deal assuming basics are ok and they sound that way.
Go to diesel forum and search under 300CD. You'll get many hits. For any mechanical questions, look at 300D's and CD's.
Windows are very slow normally. One of the problem areas due to failure of internal window mechanisms but they were slow when new.
Of course I'm biased but to me the coupes have very timeless and classy style.
'85 300CD @ 144k miles.