Don't know how it goes for BMW's, but I changed springs on my MINI once and you certainly need some patience, knowledge and special tools.
I went for the next best thing and got myself a KW V1 coilover set. I wanted to get (much) lower than what springs offer and changing to coilover set allows you to keep a lot of the comfort while still going low.
Eibach springs are a cheaper and decent way to go a bit lower
