Figure $400 to rebuild the turbo. Transmission, maybe $1500. MB sells rebuilt transmissions for ~$2500 plus installation.

Turbo problems are rare other than oil leaks into the intake stream. Lack of boost is typically a control problem. It's obvious when the transmission needs attention, less obvious if it's an external problem or up for a rebuild.

Take the car for a prepurchase inspection so you know what you're dealing with.

Sixto
93 300SD 3.0