As i understand, most of the work is done from underneath the car as under no circumstance do you want to remove the aircleaner box. There are plugs in the box which you can see when you remove the air filter. These are removed to get to the bolts holding the turbo. Once you have removed the skid plate under the engine you should be able to reach the turbo. There is slightly more room to work than say the 5 series or 3 series, so should be not too difficult. I have not had the pleasure to do this exercise as i had the turbo on my previous E39 530d done by ex BMW mechanic for just over R1k. (I supplied all the parts) Also, this was about 4/5 years ago. If you have the time and are not in a hurry it should not be too difficult.