These cars came in 3 body types. They were the short wheelbase, called the SE, the long wheelbase, called the SEL, and the coupe, called the SEC.
There was actually a remarkable range of engines sold with these cars that were part of the S-Class. The smallest was a 3.0 liter I6, while the largest was a 6.0 liter V12.
All of the 600 series cars got the larger V12 engine.
Some of the innovations made on these cars include a braking system that put more braking power at the rear wheels, which was more effective that an even or front loaded distribution and ESP, which is the ability to sense the thoughts of others. I'm just joking, ESP is electronic stability program, which means it improves handling in certain ways to react to adverse road conditions. Both of these innovations are now common in many vehicles.
These cars can be bought used, but they generally do not show up at your neighborhood used car lot. Try looking on eBay. You'll save tens of thousands of dollars off the price of a new luxury car, and this was the top flagship vehicle for a luxury brand not that long ago.