You're gonna have high presure fuel pump issues, but other then that, GO FOR IT!
EWS (CAS now) is probably going to be an issue, as BMW's in the past (from E36 on IIRC) require the key to the car, the tumbler it goes in, and the engine ECU for it. Otherwise, standalone ECU's can be used.
Another issue is the driveshaft. Most BMW's use a guibo (a big rubber donut) to attach the driveshaft to the transmission. They then have the slip yoke half way through the driveshaft to allow for chassis flex and engine movement. As the S30 is so short, you'll pretty much have to cut your driveshaft down to a one piece shaft as there will be no room for a center support bearing. People much smarter then me have said that as long as you have the driveshaft that has the CV joint end, that should allow enough play that you'll be fine.
Will it make more power and/or cost less then an LSx swap? Not even close. It's a terrible idea that no one should do.
Should you do it though? Absolutely! It's cool, and it's what you want, and it'll be interesting and infuriating and you'll learn a ton.