Sounds awesome, I can not share input on BMW as I have had little experience in viewing their suspension components, bit I am wanting to build something similar in the future. However I would like to stick with the Subaru transmission. From what I came up with the easiest start would be to build a tube chassis and build a roll cage to brace it, or buy a full cage completed.
http://ronlummusracing.com/Race_store.htm Here is where I want to buy a full cage from. However to get it shipped to Ohio prebuilt is like $3500.
I was thinking about buying a knowcked down kit but and having that be my first welding project, where the welding with be done by a professional, and I will learn along the way. Besides my brother is an excellent welder/ fabricator. I am just worried about getting everything perfect. Anyhow good luck and if I get to check out a Bmw today I will provide usable information.