If you're talking about a W124 newer is not always better. After the 2nd facelift (where it became the first E class) they changed to water-based paint, and thus began the corrosion issues. The diesel ones command a premium in the UK. I don't know why though because the right hand drive ones never got turbos. They are as slow as anything, and you're only talking about 35mpg in the real world, which with the extra cost of the car, and the fuel, is not enough of a saving. The diesels have also been used as workhorses, whereas petrol models have been looked after more. I did my sums and worked out that it would be cheaper per year and more fun to get the 300TE-24 which has a delicious and smooth 231bhp motor (more like 240 on super unleaded) if you do the right thing and get one without a catalytic convertor. It won't even get 20mpg in town, but get it on the motorway all day and you'll see over 30mpg.
It is a lovely car to drive - It's actually very modern, so has really good suspension, very good ride, decent handling for such a barge, abs, airbags in some, self-levelling suspension, and its very fast, with a beautiful sounding motor. Mine has electric seats, cruise control, and if I converted it, I'm told that the aircon would still work.
The things are built like tanks, but get one that has been regularly serviced and maintained, or you will spend more than its worth getting it back up to scratch. Front wings rust but they bolt on if you care enough to replace them.
I've had a few huge bills, but I only spent £800 on the car, and I knew some work would need doing. I spent £2k fixing 5 years of neglect, including a front suspension overhaul. The only shocker was having to spend nearly a grand for a replacement driveshaft when one snapped. But then if they only snap every 200,000 miles they must be quite good. Generally it's been faultless - all the electrics work. I've had to replace a few bulbs, and they always benefit from renewing all the old fuses. Other than that it oozes quality. It's a 23 year old car with over 210,000 miles on it, and the interior and general condition of the car look more like something a couple of years old, and the engine and box are silky smooth and good for another 100,000 miles. This was a £40,000 car in 1990.