Simplicity - a 240D with the simple A/C control knob (not the rotary dial; I've heard this is more difficult to work on) and manual EVERYTHING.

If you want something a little better/complex/more creature comforts, I've read in many places that the 1985 300D Turbo is the model to get.

Personally, having driven all W123s except for the 300TD, I would say that the 240D 4-speed was my favorite. The 1985 300D Turbo I drove was great too, but the 240D with a stick shift just has this good nature to it that's hard to describe. It's almost like it's everything you need, and nothing you don't.

I didn't like the 300D NA, because it had a 3-speed automatic and it seemed to have pretty high freeway RPMs which were bothersome. The later models have a 4-speed automatic.

Drive which one you want. Pick one based on condition overall. Be happy.