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.