I'd recommend the ED80 as well, they come up from time to time second hand as well at bargain prices. Get the skywatcher reducer flattener and make sure you stay with the stock focusser as that will be needed for the Skywatcher reducer. The stock focuser may need some tuning, but it's easily done. There is also a wider T adapter available (Bintel have them) which will allow you to use the 5DIII sensor to its full capabilities. You mentioned M42 here is an unremarkable image 50% crop) taken with the ED80 SW Focal reducer 5DII.
http://www.iceinspace.com.au/forum/s...d.php?t=111457 and one of M31 taken with the same gear uncropped.
http://www.iceinspace.com.au/forum/s...d.php?t=111455