I don't know why it couldn't be done in a W123.

I'd remove the top end of the engine first (cyl. head, manifolds, etc).

I'd remove the radiator and the fan but leave the oil cooler in place.

Now I would inspect each cylinder to see if there is any MAJOR damage like deep grooves or scoring. I'd do that by turning the engine over with a socket on the crankshaft bolt and try not to get the timing chain jammed up. If it does, just turn the engine backwards a bit and pull the chain loose by hand.

Then I'd remove the A/C compressor from its bracket, although I'd leave the A/C lines connected, and hang it from a wire so it wouldn't put pressure on the A/C lines.
I'd do the same with the P/S pump.

I would then remove the large allen bolt from each motor mount and jack up the engine slowly as high as possible with a floor jack under the lower metal oil pan (with a block of wood between jack and pan to not damage it.)

Then I would put wood blocks between the rubber motor mounts and the mount arms to support the engine and remove the floor jack.

Now I'd remove the lower small steel oil pan.

Then I'd remove the upper oil pan. It will probably be a tight fit. You'll need to pound out the oil dipstick tube with a homemade rod and probably rotate the crankshaft with a socket on the crankshaft bolt to get clearance.
What I mean is that the pan will come halfway out then appear to be stuck. It's just hitting a part of the crankshaft. Rotating the crank should get you the clearance you need.

On the bright side, if it's an 85 300D CA model, now you can get to the starter easily and rebuild it!