I have a modded my 335i as such I'd recommend:
Ditching the bmw performance power kit and opt for a Piggyback tune (JB4/Procede) or remap. Running meth (as I do) you will need a piggyback to control/optimise the flow.
BMW performance suspension would likely be more expensive than Eibach pro springs and Bilstein Shocks which work well. A full coiliver setup like Ohlins would be my first choice though.
AP brakes will do nicely!
Consider a Quaife ATB differential (I wish I had one) -
M3 suspension arms and subframe bushes.
Its never ending!!!!!
With these mods, an E60 M5 would have trouble keeping up!