Would he also need to replace the tranny? I wouldn't think a 318 tranny would mate well (if at all) with an M3 engine.
You should really consider getting the M3 instead, even if money is an option. As it would all balacne out in th end. Plus there is a bunch of things you could do to lighten the M3. Remove spare, carpetting, swap parts out for CF parts, etc.
P.S. - or do like Donut did in his ///Mx6 and remove all the seats, interior, and the wheels. His car moves like greased lightening now. I saw him beat two Vipers and a Lambo. No really.