Is it possible to change a 300D non turbo to a turbo without doing some serious engine work and is it economically wise?

It is possible to convert you N/A diesel to a Turbo Diesel, but it won't be cheap. The gains you get for the money arn't really worth the cost. The only exception is if you can find a wrecked 300D with a turbo and scavenge the turbo and associated hardware off it. If you need more info please ask. I and doesn't disappoint you too much.

As I understand it the turbo engine is updated with high temp valves and a hardened and/or oversized crank to handle the increased engine torque and combustion temps. I would recommend to just find a decent turbo engine and swap the whole thing. I hear you can find them for as low as $500 sometimes.