Hello PJTN. Here's another opinion FWIW.
I'm using a 5DII and printing to 24” on an ipf6300.
With the caveat that I've only gone to two minute exposures, the 5DII is completely up to the job you want to do. You will see no difference on a 13 inch print between the Canon and a medium format back, and will be hard put with a 20 inch print to see any difference.
The problem is the 24-105 lens. It is not up to the task. It wasn't until I made 24 inch prints that I realised how really bad it is. I've purchased a Zeiss Contax 35-70 with Haoda adapter (for about US$450 all up) and the first comparisons with the 24-105 were absolutely astonishing. So much sharper, better contrast, better colour. The drawback is it's manual exposure, and although the adapter has focus confirmation I don't find it reliable so live view is a must. Also if you go down the adapter route be aware that if the adapter is not well made then it can short out the battery and your camera can burst into flames, or the mirror can break, or it may not be aligned and won't be sharp across the frame.
What I'm saying is that there are some nice Canon and Zeiss primes, and some new zooms in the pipeline, and I think other options would be a waste of money at present. Most folks aren't realising the potential of the 5DII because of poor technique, poor tripods, heads and so on. I also see a noticeable difference when I shoot with a 2kg beanbag on the camera. A pain to carry around though.
Nice photo by the way.