The best way to do it is by using a 256 degree Schrick intake cam and moving the current m50 non-vanos intake cam to the exhaust side. I'm sure you can get custom software from either AA or Turner. Don't mess w/ s50 cams they won't work in the non-vanos heads.
By the way swapping the Schrick and m50 intake cams in the non-vanos head will result the same lift and duration as the m3. Thus it is the same as doing the s50 cam swap.
I don't know if turbo is best for our engines, however you can make one bad ass n/a 2.9l stroker. All you need are 1. s50 crank 2. s50 rods 3. s50 pistons and 4. slight bore of the block. Supposed to produce between 230 and 250 at the wheels depending on what bolt ons you have.
Hope this helps,