PLEASE specify if you are referring to early 300D (W123 chassis, 1977-1985) or the later model 300D (W124 chassis, 1987-1995)! They are TOTALLY different cars with TOTALLY different "problems", and they're both "300D".
But for either, the turbocharger itself normally lasts forever. The engines if cared for properly are usually good for 300-500kmi. The later OM60x engines are more fragile due to the aluminum head, but will last just as long IF they are maintained properly (particularly the cooling system).