So this guy is selling a car with a 'bad' engine (whatever the reason is), and he is telling you what the DEALTER TOLD HIM what the problem is.

And you're taking his word for it?

Major mistake.

If you are buying a car with a 'bad' engine, it is YOUR RESPONSIBILITY to know EXACTLY what the problem is. If you are not confident that you can diagnose exactly what the problem is, they you should pay to have someone diagnose the problem FOR YOU. In fact, because this may be such a serious problem, I'd have two separate shops give you a diagnosis.

Although there is a slim chance that this is a minor problem, most of the things that would cause this problem are all pretty bad. The least bad of all of them is the turbocharger (because the turbocharger is external to the engine). So, I find it suspicious that the sellor is telling you the problem is the turbocharger.

If you buy this car and it turns out that the problem is not the turbocharger, it will be your own fault that you did not do your appropriate due diligence.