This thread has gotten a little confused, maybe I can help a little.

The 300D is a "midsize" (like the current E-series), and it's a W123. They were made with a non-turbo engine from 77 through 81, and a turbo engine from 82 through 85. The exception is the W123 wagon (300TD) which got a turbo in 81. The 77 through 81 (non-turbo) cars are very similar, and the 82 through 84 (turbo) cars are very similar. The 85 has a few changes that make it unique. It has a higher final gear ratio that improves mileage. It also has a somewhat different automatic transmission design. There are a few other details in the electronics that are different. The 85 300D cars that were sold in California also had quite a bit of additional pollution control equipment, and those cars are usually not considered desirable. The 85 California cars can be recognized by their large "canister" type air filters. As for the "best" and "worst", condition is more important than model year. If you want a turbo, you are limited to 82 to 85. The 82 through 84 models are essentially identical. The biggest advantage of the 85 is the higher gear ratio, the biggest disadvantage is probably that there are less used parts available for the unique pieces. My advice is to look at the condition and maintenance history, don't worry about the year.

The 300SD is a "full-size" (like the current S-series). The "old" style 300SD is a W116, made from 78 through 80. The "new" style is a W126, made from 81 through 91 in various versions. From 78 through 85 the 300SD used the same 5 cylinder (617 turbo) engine as the 300D. After 86 it used variations of the 6 cylinder (603) engine. I'm not an expert on the W126 300SD, but I don't believe there is not not much difference between the 81 through 85 models. Again, I would pay more attention to condition and maintenance history than model year.