Turbo's on diesel engines are generally extremely reliable. While they can fail, it is rare. Outside of some foreign object flying down the intake, the most common turbo killer is lack of oil or dirty oil.
Basically, you don't know what happened, and you can't rely on anyone else, so I would recommend that you find the best diesel mechanic you can afford and pay him about three hours labor to look it over. It is obvious that the previous owners took the cheep way out on the fix, and they probably took the cheep way out on everything else.
If you end up with this motor it will probably be cheaper to leave it as a turbo. Change the oil at least twice as often as the manufacturer’s recommendation. Everyone wants to extend the service intervals on engines, but it is a bad idea and the people that are recommending it do not own your engine or have to repair it.
Concerning an earlier post, the engine is not currently running rich due to lack of turbo pressure. Turbo boost comes on and off continually as the throttle position and RPM and load change. The fuel injection compensates for manifold pressure to keep the fuel to air ratio correct at all times, even when there is no additional boost provided by the turbo.
Good luck with this engine. Be very suspicious. It gives me the willies.