You want to have one of the following:
Leaf Valeo11
PhaseOne H10
Sinar 43
Eyelike M11
All 4 of them have the same DR as any current digital back, eg. 12stops. All four of them are 16bits in color capture. They will all blow any DSLR, including the 5D, out of the water IQ-wise when used under the right circumstances. They are all around 11MP with a 4:3 ratio.
They are for sale sometimes on Ebay or second-hand (check out Calumet second hand) and cost around 2K to 3K USD.
Mind you, none of them have a display. Only the leaf can be used without tethering by means of a digital magazine. All of the others need to be tethered to a computer. The Leaf can only be used with a Mac. The others can (as far as I recall) be used with both Mac as well as Windows.
There are other more exotic things (Fuji Luma, Dicomed Bigshot, S3, etc.) however these are no longer current. The models I mentioned above officially are not taken in for upgrades but most manufacturers in reality will.
Another option, when you want to spend a little bit more, would be the Imacon 96C. This will cost you between 3K and 4K. Advantage is that it is a square chipped back (37mmx37mm), 16MP, DR 12 stops. It has adapterplates so it can be used on other systems as well. It has a display (the older ones are monochromatic) and it can be used with a digital magazine for portability. These appear regularly on Ebay
Why didn't you put this on the MFDB section, there are a lot more knowledgable people over there. By coincidence I noticed this topic.