I would welcome your advice.
I am contemplating getting a used Canon EOS 5D (mk I) as they are so cheap at the moment. This may lead to getting the Mk III at a later date, but I certainly won't get Mk III to start with as I'm just dipping my toe into the Canon system to see if it suits me better than my current setup.
I don't really want a discussion on whether my current system is better or worse than Canon or we'll be here forever and all the uber Canon devotees will only mock my current line-up in the way Ken Rockwell pours contempt on whatever is not his current "best camera ever"!
So, to the point; in terms of image quality, is the 5D (I) similar to the 5D(III) apart from the image size? I am aware that the 5D (III) has almost double the Mp. I am interested in using it primarily for hand held available light work. I would be happy with 12Mp and would more than likely shoot RAW. If I was to use a 5D (III) I would use the reduced size RAW option for this kind of work.