I just know it won't be an Alpina engine...
M4 engine would suffice. There are (IMHO) 2 problems with BMW turbo engines - firstly the noise isn't quite there yet, both the M3/4 and M5/6 sound a little awkward - like they are making up for the lack of induction noise and the turbo sucking effect by creating deliberate pops and bangs after the rear silencer. The second is the fundamental benefit of turbocharging - the torque curve. A big chunky flat torque curve is great fun for a while, but I became bored of it quite quickly and prefer a more linear delivery (as per the 4.8 or a supercharger). Interestingly the Ferrari 488 is mapped to deliver just that.
