You can put an aftermarket (or stock) turbo onto a NA 300D. Many people have done it. The problems are fairly obvious -
higher stresses may lead to crank problems - do not go overboard on boost pressures or fuelling increases.
High EGTs may cause piston melting issues
etc.
It then depends on how you drive the converted vehicle. Treat it like a sports car
and it will have heart failure - sooner or later - probably much sooner!
Treat it well, and it should continue to give you excellent long service.
I would consider an EGT gauge mandatory on a conversion of this nature.
Tony