Being a modded DSLR imager and getting some results that I'm happy with, I recommend that you mod your DSLR and get to know AP with it before hand, especially when you're planning on going mono... buying a OSC camera as a stop gap seems like a waste...
But I wouldn't mod a camera for the price you have quoted.. I modded my DSLR myself and I'll tell you that the job is not worth 300+ pound.... if you don't want to mod it yourself, than there are services that are 1/10th of that price like quoted by @carastro...
Later when you go mono, you can always use the DSLR for your color data, and use the mono as luma for detail.. that will cut your imaging time substantially not needing to expose RGB individually...