A lot of stuff will have to be checked out. The boost line is probably blocked with crud for the 0-60 times to be that slow. Or the turbo is bad or the linkage to the injection pump does not have the full range of movement it should. Or fuel supply or pressure is inadequate.

Personally I would do a compression check on the engine as the first thing to make sure the effort is worth it and you do not also have a cylinder problem. This avoids flogging a dead horse.

Otherwise it just sounds like an older unmaintained Mercedes diesel. Perhaps most are several small things wrong that are more labor intensive than cost intensive. I also wonder if your egr valve is stuck open?

That and the boost line are worth the checking effort prior to the compression test. Some of these engines have signifigant blow by. Of course that is not the best. Yet to me the important thing is the amount of miles per quart of oil. Also I assume you have had a look at air filter to make sure it is not saturated with oil?

A new standard I have come up with of perhaps an indicator of what is actually causing the blowby. We always assumed it was only the rings and bores. Old tired valve seal rubbers that have pretty much disentigrated with time and worn valve guides will not make an oil burner of these cars in themselves. Yet they can increase what we call blow by.

If you think about it they can add pressure to the crankcase. The turbo keeps the pressure at the bottom of the intake guides at a positive pressure Where on a gas car they are at a constant vacuum or negative pressure. The bottom of the exhaust guides are usually at a small positive pressure as well. So rather than sucking oil and air down them they can instead add pressure to the crankcase.

That is why I use the premise of how high is the actual base oil consumption. The valve seal rubbers did not last thirty years on their gas cars but most of our cars still have whatever is left of the original ones still in there. Since the diesel valve guides also might suffer a little less lubrication than the gas engine guides. After a couple of hundred thousand miles one would wonder what shape they are in. The engine in general is not a real high reving one so even with a poor condition of them existing it will still seal the valves well enough.

Not increasing oil consumption that much at the same time. Conversely if the oil control rings are tired you will burn more base oil for the same amount of blow by.