Outstanding write-up! It has been some years since I have replaced a W123 starter and this thread is a great refresher for me.

One observation I'll offer:
Where there is discussion in the thread to "Rotate engine clockwise by means of 27mm socket attached to crankshaft bolt at front of engine...", I would add:
First, place your vehicle's transmission shifter into the "N" position.

Thanks for this great DIY post.