I have seen several nice W123 turbos and W124 2.5 turbos on Fleabay, and I found an 87 W124 with 155,000 miles on a car lot. Haven't seen any service records on any of the cars. Having owned several Benzes including an 83 300 SD I know the service records of the car is a good indicator of potential repair problems. I know the 300D 5 cylinders are bullet proof, but they are noisy. Are the 1992 2.5 and the 1987 3.0 turbos quieter? And do they get better mileage than the older cars? How dependable was the 87 model. With the newer cars I also get airbags and ABS which is a plus. TYIA for your input.

All depends on how the car was taken care of. The 124's are a little quieter and start in below 0, where my 123 in 18 deg weather gave me a problem. Out of the 124 engines I would go for the 2.5 turbo. Lot less problems with the head. There are some people who have 124's with the 3.0 turbo that have had luck with them, but all in all they are very problematic. I have had three 123's and one 126. Engines are bullet proof. The only thing I had to do with it was replace the turbo on the 126, "My fault for putting a K&N filter". I'll tell you that my 124 2.5 turbo is a lot nicer ride and feels like a brand new car. Lots of pep and 32 mpg. The best 123 I had was a Euro 300TD, that I sold on this board. That gave me 28 mpg. So they are both great cars. I like the 124 better.
Rob

124s handle better, but the 3.0 turbo-six has a weak cylinder head that frequently cracks if overheated(it's also the fastest of the 124 diesels). Also, you gotta love that 124 evaporator issue, and the engine wiring harness(which on the diesels isn't as bad as the gas cars). Other than those and the fact that they are not as easy to work on(although DIY is far from impossible) they are superb and I myself want one super bad but can't find the right one. The 2.5 Turbos have a weak oil pump chain so be mindful of that.
123s are awesome. But they are old. And they smell like crayons. They are the easiest to work on that I have ever seen. But they look quite dated and that might not appeal to some people. If you obsessively maintain them who knows how long they will last.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)

I have never been dissapointed with a w123, and I personally think they are more attractive looking.

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1998 E320 150,000
1980 300SD 240,000
1965 190D 79,000

Buy whatever one is in the best condition with service records. You don't want a used car lot pig!

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200