The rear C pillar/window frame is going to be the hardest part. Only place you can get it is off of an M3 parts car.
This thread will give you a good idea of what the process will be:
E30 M3 Steel Bodywork Conversion - R3VLimited Forums
BTW, you might want to use an early E30 ('87 and older) for the conversion since they have the correct tail light panel already.