re: the 560, it's a big 5.6 L V8 that was designed before fuel economy was a major selling point. 15 mpg is probably close to the best it will give.

300E's should give somewhere in the range of 15-18mpg in the city, and around 18-22 mpg on the highway. Some examples may do better on the highway, I used to get 24, and I think some 5-speed owners have gotten up to 27 or so. Not spectacular, but also not terrible for when they were designed. I've been looking at newer model vehicles lately, and the bump in gas mileage is on average only a 2-4 mpg, despite 25 more years of technology. (although, that does sort of come out to something if you look at it on an annual basis.)

These cars are somewhat reliable - they will last for a long time and there aren't too many things that will leave you catastrophically stranded someplace. But, given their age there will be a lot of things to repair on a regular basis. They are definitely not maintenance-free vehicles.

Anthony