I would do it like they do in E46 or E39
Keep original B25 wiring & DME, but use the B30 intake, MAF, injectors, throttle body. The coils, coil-wiring and valve cover should be the same. Get it running as a B30 with the B25 tune. It should start and run well.
After you get it running, you can consider an update the DME Software.