The 1979 SD and the 1978 CD both have the earlier Type II climate control. More complicated than the later system, more difficult to troubleshoot and some components are pricy.
The SD is a w116 chassis. Nice car. I used to own one.
The CD is about the same as other w123s, with a few differences. A '78 will be a non-turbo so value/resale is less than a nice turbo version.
Can you be more specific about how the 240d is noisy? All the time, at idle, on the road? A bit more entertainment is to be expected, as you're hauling nearly the same mass around with one less pot and no turbo, compared to a later 300d.
None of these cars has any inherent faults (the Type II AC is a minus but not a fault). When dealing with old cars such as these, each car has to be evaluated as an individual unit, not what model it is.
If you are going to see them... we would love to have pics upon which to comment....
