Valve adjustment on 92 300D 2.5 Turbo

My mechanic told me it is never necessary on this car...is that right?

[dieseldiehard]

Yes, it uses hydraulic lifters. Think of the savings, not having to have this done every 15K miles will amount to thousands of $$ over the life of the engine, or if you DIY the mess and time avoided is considerable.
OTOH, you should be using synthetic oil, the extra cost of that needs to be factored in but if you buy in bulk such as AMSOIL you will offset the extra cost.