1) No, it's an outdated back with a smaller square chip and it's no longer supported by the manufacturer, so any repairs might be tricky/impossible.
2) No, your 5D is an overall better camera.
3) Buy a FF 22mp or newer if you want to step up from your 5D, like a P25 or Leaf Aptus 22. Buy it in Mamiya mount and you'll have access to plenty of bodies and cheap lenses on the used market.
It seems that everyone is saying no...but then they're recommending various 22MP backs which are only 1/3 greater in sensor size and pixel count, and of the same 9-micron CCD design generation, but twice as expensive! That advice, to the OP who is up front about being budget limited, doesn't make much sense to me.
Also, some advice is plain factually wrong:
"
You will need to tether" - no, that was the whole point of the Kodak DCS 645 series: they broke free from the obligation to tether, whether it be to a computer or to a bulky ImageBank hanging off yourself, and gave self-contained DSLR-like operation: CF card, Li-Ion battery, colour LCD display. (But they still support tethering for those who want it).
"
any repairs might be tricky/impossible" - no, Midwest Camera Repair have all the parts (very cheaply) and servicing (ditto).
As someone who actually
has both the 5DII and the Kodak-Mamiya combo, I say (a qualified)
yes - I already laid out my arguments in favour of the Kodak-Mamiya combo to the OP, plus many sample photos, including
Infrared, when he posted the same question on Pnet.
Have a look.
Ray