Which turbo was on the W124's?
 

My unit is a Garrett, but which one? I saw Kent's YouTube video and want to refurb it before it has any problems.

1991 300d 220k

Kent's video? If it aint broke don't fix it.

What 300SD said. Unless the compressor wheel is torn up, it's leaking oil, or not boosting properly (assuming good fuel and wastegate adjustment) leave it and get as many miles out of it as it'll give. They don't fail from use, they fail from poor engine maintenance or sucking in debris.

I think it is leaking oil
 

Not sure what 'leaking oil' means, ie where that would leak from or to but a mechanic told me previously that it was leaking oil.

The exhaust/turbine wheel has a shaft that extends all the way out to the compressor side and the Compressor wheel slides over that and is held it place with a nut.

There is an Oil Seal on the exhaust side of the shaft and the compressor side of the shaft. When they are worn out either or both leak oil.

The Oil pressure line going in on the older models have occasionally cracked. Leaking Oil.

On the bottom is the drain back to the pan. Those have also been known to leak on the earlier models.

On the exhaust side of the turbo you remove what is needed to see the turbo exhaust/turbine wheel and if the carbon looks shiny or wet it can be a indication the Oil is leaking out of the seal. If it is really bad you will have blue smoke

Because the Turbo usually pulls in the blow-by from the engine it can sometimes be harder to determine if there is a oil leak on the compressor side or not because the compressor wheel will have a light film of oil on it from the blow-by. If you see any pool of oil inside they you can expect a leak or the mentioned blue smoke.