Water pipe isn't to hard a job if you use one of the aftermarket pipes. I prefer using the one that has the snap rings that engage and lock the pipe in the expanded position. I've heard about people having issues with the threaded ones and have seen them fail prematurely. The alternator housing gasket is a little more work to get to, you aren't really working around that area of the engine based on the repairs you are doing. Also depends on how much you tore down to do the jobs you're doing. The more you tore down the easier it will be to knock these two out.

To second the point made by X53Jay4.8is, if it were my X and i tore it down I would do it. When I bought it my X the coolant pipe hadn't been done, I was in school at New York and it let go when a good friend was driving it. Had it trucked back down to GA where my indy did the job for me. When my sister got her N62 545i we did a complete 100k service as well as all the gaskets including the alternator housing gasket and the water pipe. They're both known issues with this engine so may as well handle them now.

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2004 BMW X5 4.4i-Sports Package-Premium-140,000 Miles