When I did it, I went to all the different Mercedes forums I know of, used the search function to search for "M103 waterpump/ 300E waterpump/ W124 waterpump" and printed the best, most clearly written ones.
I took a weekend off, during Thanksgiving weekend, and replaced with over the course of the weekend while watching college football.
If you have a modicum of mechanical knowledge it should be a decent DIY job. Fortunately, Mercedes from back then are made to be repaired. It's pretty straightforward, pretty clear. There are some hidden bolts, and you will need a series of universal joints and wobbles extensions. There is one bolt that some write ups said required a special GM tool. I bypassed that by using a quarter inch drive long extension and the requisite adapters
To whit; print the clearest, most concise directions you can find, paste em up on the wall near where you are working, make sure that you understand what they say, and take your time.
It would behoove you to replace the belt tensioner shock and the belt while you are there
'O=00=O' BMW 2002. long live the legend.