240D is pretty tough. Sometimes you run into vacuum problem if the vac pump fails. But if reliability is measure only in "dispatch rates", then I guess you can still drive it with no door locks, possibly a poorly working climate control, and having to shut the engine down manually!
In the more modern era, I find the C230 a really good car, a little better in the earlier non-supercharged version, because we do have occasional engine management problems with the supercharged version. But to be fair, these check engine lights most often won't affect "dispatch rates" of the car any more than a vacuum system gone south on a 240D.
The C230 looks like a lot more reliable car when comparing the amount of convenience and safety equipment on the newer car, but I realize it's not really a fair comparison. Although, the 240D and C230 were both entry level cars.