I agree, no special skill set to rebuild BUT the advise you get is critical. I have spoken to so many people who say that a 30/76 would be great in the M35. Jetwreck is running a 30/71 in his car and his car has a lot of lag.
I just had mine rebuilt as a Sierra Stage 1 (compressor =43mm / Exhaust = 45mm) and this is almost perfect IMO. There is some lag but not an issue to drive around but it comes on strong and what is equally crucial is the question, Is this the last mod you want to do?
The reason I say this is because my injectors are at 100% duty cycle at 6800rpm (or maybe a tad higher), so any bigger turbo will mean you need to think about new injectors and piggy back ECU.
Cheers
Andy