Looking at a 76 300D in excellent shape
 

Low miles ,no rust. I have had a 240d and a 220d. How does the 300D compare ?

More power (not enough, just more:)),
Less room in the engine bay,
Key on/off vs. push/pull knob,
Nicer interior appearance than 240D, wood dash appliqués,
Power window???
Power antenna???,
Quieter ride inside from engine noise due to better firewall insulation IIRC,
Bigger tires and wheels IIRC,
Possibly stereo vs. mono radio
Slightly less fuel economy, and
A little more pleasurable to drive around town.

Good luck!!!