Here is a How To from Bimmerboard - same engine (if you have a V8)
How to Replace Alternator in a 99, 00, 01 740il
Id get 3 gallons of antifreeze. Its not a bad job, a bit tight to get it out, and it's pretty heavy - just take your time. I used a piece of cardboard to keep up with the bolts that hold the alternator in, as they are different lengths. Just rough sketch the pattern and push the bolts through, cant go wrong.
There may be a DIY on here, but I haven't looked. Try the search feature - Good Luck! (wear gloves to save knuckles)