I can only comment re my own car which is a 1988 W124 300D with 142,000 miles on the clock.

The springs on the front of my car are getting pretty saggy and will have to be replaced soon.....I can just about get 2 fingers between the arch and the top of the tyre. Other than that the car is still smooth to drive.

Diesels tend to be heavier on their suspensions than gas engined W124's due to the weight of the engine.

I would say that with your low mileage your suspension probably still has some life in it

If you are getting any clonking from the front, its likely to be your sway bar bushes which are cheap and easy to replace....just done it on mine and its now real quiet with no clonks.

I'm not aware of the Diesels ever being fitted with Sportline suspension

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Current Mercs
R129 1994 300-24SL
W124 320E Turbo Technics Twin Turbo 350bhp

Previous Mercs
W124 1986 Quad Cam AMG Hammer, 45,000 miles
W124 300E Turbo Technics Twin Turbo 320bhp
W124 E500