Please excuse the stupid question but I haven't had much luck finding an answer on my own.

What are the major differences between the mid 80's 300D and the 300SD?

The 300D is the W123 chassis and the 300SD is the larger W126 Chassis, and the early ones were thre W116 Chassis. Now the differeces are that the W123 is much smaller and have the two 7" headlights with round foglights. They are also much simpler and don't have power seats. The W126 is alot larger and has power seats and alot of other luxury features that the W123 does not have. Both however use similar turbo engines, except for the non-turbo 300D's. The 300D's are much simpler, so they can be less expensive to repair if you can do a lot of the work yourself. There is more that others can add to this.

To add to what's been shared...

75-76 300D has the boxy design from the late 60s, no turbo
77-81 300D has the more modern design, no turbo
82-85 300D same design as the 77-81, turbodiesel
78-80 300SD same design as the 450SEL, turbodiesel
81-85 300SD same design as the 500SEL, turbodiesel

All these cars have the same basic engine.

My opinion is that outside of power seats in the 81-85 SD, an SD isn't more complicated than a D. In fact I read about fewer problems with the SD's dedicated central locking vacuum pump than I do about the D's central locking that shares the engine's vacuum pump. The AC systems are similarly flaky.

It's easier to find a D with MB Tex instead of leather. MB Tex holds up a lot better than leather does. I've never seen an SD with MB Tex but I've read that they exist.

Sixto
91 300SE
87 300SDL
83 300SD

So California gets more emissions junk and leather. There must be millions of freezing cows somewhere will all the leather I see in SDs around here.

Sixto
91 300SE
87 300SDL
83 300SD