Stick with the factory size recommendation: 195-70-14.
They look right and work well on that wheel. Check out the Michelin Rainforce MX4 tires, I've had them on my 85 300D and my 84 300SD and have been very pleased. I think you can expect a treadlife of about 50k miles on your 300D with those tires....
1984 300SD 140.6k