I finally got both the injectors changed in the 260E 124, and 300SE 126.
On the 124:
I noticed a prompt response on the kickdown, and the lightness in touch on the gas pedal. Previously, it was sluggish. Now it behaves more civilised with an even-power distribution at torque. Relatively ease in projecting 0-100 km/h without the sudden surge and slug.
The engine sounded smoother too - not so much like a diesel powered powerhouse.
I felt it was worth the while replacing the injectors - despite the advise of some.
On the 126:
The noticeable difference is in the accelerator. It seems much lighter than it was before, and the delivery of power could be felt - smoother than it was and with a gradual silky acceleration. Response was more sensitive on the depressen of the gas pedal.
I also changed the brake-pads (had original, and refitted original). Applied the brake paste AND cleaned with a generous spray of Brake Cleaner over the disc and within the brake-clips. Noticed silent-braking now. Am wondering how long would it remain silent before the squeaks return.
It WAS necessary to change the injectors.
Thank you all for your comments and advise, to which I am grateful and guided.