Not hard if you are mechanically inclined and have access to some good tools. There is a lot of similarity bet the E85 and E46 suspensions. If you dont get in a rush it should be pretty straightfwd. Only tricky part is spring replacement. Not to insult your intelligence but an alignment is needed once its all done.
Here are a couple DIY's
http://forums.bimmerforums.com/forum....php?t=1096399
http://forum.e46fanatics.com/showthr...aliper+rebuilt
Note on the 2nd DIY he takes his struts to a mechanic who has a hydrualic spring compresser. Not saying this is the only way but spring replacement is the big dog in the DIY. If you have not done them before, threaded rod spring compressors can be difficult to dangerous. I think getting access to a proffessional tool is the way to go.
Do a little searching on this site and the E46fanatic.com site for the Bilsteins. Both have suspension sections in the forums. If you havent done it already you need to decide what you are after:
1. BUDGET(600 to 3500)
2. Ride (comfy to chiropracters best friend)
3. Use(DD, track or both)
4. Height(slammed or stock HT to mildly lowered)