Do you have any idea about potential power level of 132 kW (180 HP), 700 Nm MAN truck (TGL) 4.5 litre engine with damaged (not working) turbo?
As a rule of thumb, for every atmosphere's worth of boost, you'll double the amount of power the amount the engine puts out naturally aspirated. For instance, if an engine puts out 100hp naturally aspirated, the same engine should be able to put out 200hp if you boosted it to ~14psi (air pressure at sea level is somewhere around 14.7 psi).
NOTE: This is considered a rule of thumb and does not equate exactly to the real world. You also have to figure in parasitical drains and other factors when computing these numbers.
With this in mind, if you lost the turbo on such a truck engine, you can assume the engine is losing major amounts of power/torque due to the loss of turbo pressure. Production diesel engines usually run in the neighborhood of 15 to 30 psi of boost, though I cannot find an advertised amount for this engine. Even at 15 psi, you can assume a loss of at least 1/2 of the power production, which would put it down to the 90 HP range and the torque down from around 572 ft lbs down to 286 ft lbs (guessing a 3000 peak torque range).
Would it accelerate to 55 km/h with full load (8 tonnes) smoothly without any problems?
Remember that it's torque which is more important to trucks. Losing 1/2 of the available torque will make it very difficult for the truck to accelerate to any given speed. The truck will take twice as long to accelerate than normal, if it will ever get to speed at all. 55 kph is not very fast in the scheme of things, but even so, you will have problems getting up to that speed, let alone doing it smoothly. You are not doing your engine or truck any good by trying to run it at this diminished level.
When a turbo charger in such diesel engine breaks down, does it break down suddenly or gradually with power levels fluctuating (ie. halting the engine, then revving it to 4000, then halting again with enormous level of dull smoke going out of engine / exhaust)?
This is typical of how a diesel engine runs when the turbo goes out, especially the voluminous amounts of black (sooty) smoke. The reason for the smoke is, the engine is not taking in as much air as it normally would, but is getting the same amounts of diesel. Not all of the diesel gets burned, so it comes out in the exhaust as soot.