Since my Monochrom order had been taking so long to come in, I canceled it. That and I knew the M 240 would be fine for B&W, so why bother? Then get a call a couple weeks later to say the Monochrom was in... so I rushed to pick it up. And love it. The High ISO is just wow - but I have had some horizontal banding at ISO 10,000 in a very dark lodge/pub/saloon.
So then I wanted to cancel the M 240 pre-order, but couldn't - it arrived and my deposit wasn't refundable. So I sold some more things, including almost all of my Canon gear, and now I have both.
The ISO isn't as high or as nice looking on the color images on the new M - as expected. The B&W is pretty close though, especially converting from RAW.
I know your wallet doesn't want to hear this, but the combo is great. Can use the same lenses, no crop factor, best of digital B&W (IMO) and a solid new, quiet shuttered, long battery-lasting color image machine. If I could have canceled my M 240 order though, I probably would still be happy as a clam - Monochrom for B&W and M7 for color film.
Rambling aside:
Yes - you should like the Monochrom very much. Especially for the high ISO stuff. And you shoot mostly black and white - this is the best B&W experience available. Period.
No - you shouldn't worry about a color body right now. That 20% you use for color? I think the Monochrom will keep you too enthused to worry about it. Get a Sony RX100 if you really need a fallback.