What do you want to know?
The engine is a hemi fashioned twin cam 2 valve per cylinder unit with EFI. It is revered as perhaps the most durable I6 Mercedes has ever made.
In Euro form, it puts out 185 hp @ 5800 rpms, and 177 lb/ft of torque at 4500 rpms. So you won't feel the most power until you're past 90 - 100 mph.
The S-Class 280 gearing was 3.46 :1. Not sure about the Euro W123.
I used to get 20 - 21 mpg on regular unleaded in the W126 280SEL I had. This was mostly highway driving.
The top speed potential is higher than an American 500SEL.
Top speed of a W123 sedan 280E is 124 mph. Man would I LOVE to see a W123 sedan going that fast, lol!
In fact, my dad's got one on the lot that I can play with in the future. It is US spec, however, but it is an M110. Imagine a W123 body with a US spec speedometer of 140 mph and no tach. Ratio of gearing in the US spec is short. 3.69 :1 in top gear.
Ah, but that 6500 rpm redline was so fun to listen to and peak everytime I pulled out on the open road.