I don't believe the M9 or any current digital back would be the right tool for the assignment as you've described given the options in 35mm DSLR's.
Noisewise, the M9 is an improvement over the M8 but falls short of any current pro or prosumer body from Canon or Nikon if you're specifically looking at
low light, hi iso as an important criteria for your shoot.
I did a direct lowlight comparison last week between an M9 with 24 1.4 asph and a 5D Mkll with a 24 1,4L Mk 1 shooting inside of Notre Dame Basilica
in Montreal.
Up until 800 iso one could flip a coin and easily confuse both.
Beyond that point, the 5d Mkll starts pulling ahead, more so with each increase in iso
That's not to say the M9 performs poorly beyond 800 iso only that it shows more noise.
I didn't shoot the D3 at this time but I did the same series of shots about 6 months ago and wager that the D3 starts pulling
ahead of the 5D Mkll at about 1600 iso but, of course, you have now dropped to a 12 Mp capture.
My 5D Mkll seems to be about 1 stop 'cleaner' than my 1Ds3 so a 5D Mkll might be the way to go
Mark
www.marktomalty.com