My car is 23 years old and has about 75000 miles. From my reading good reliable suspension parts are getting hard to source.

When might I expect failure to begin?

Should I try to source good parts to have on hand? It appears good parts will become increasingly difficult to find.

It appears that most of the available new parts are not as good as the Mercedes original parts. Should I replace only when there is a failure or do all related parts at one time?

Is there a coating that can be sprayed/brushed on the rubber parts to reduce failure by age? It seems if road oil and ozone can be kept off the rubber the parts might last longer.

From my reading manufacturer parts seem to be best, then maybe lemforder, Meyle-HD. Any brands that can still be trusted to provide good service?

This is a baseless concern. Body parts are getting tough. Exhaust parts, too. But suspension parts, other than perhaps self leveling stuff, still common. As for aftermarket parts, I'm running Kyb shocks, Dorman front lower control arms, Lesjofer springs, and a no name set of rear axle bones on my 190D, no problems. The youngest component of this lot has already seen more miles than your entire car. Most of these parts interchange with your 300D, the suspensions are essentially identical.

Yes and a lot of the rear stuff was brought to the W210 as well.

Parts very easy to find still.

Can’t go wrong with Lemforder.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech