1. Go to a shop
2. Try a 5D mk II (if still available) or a 5d mk III. Rent one for a day if possible.
3. Have fun.
4. Do you like the images?
No! -> don't buy a 5D
Yes! -> Do you need the extra features of a mk II or mk III? ( eg. Auto ISO, more C-buttons, video,...) then buy mk II or MK...