Here is a ff that I took today - uk otter
Lovely shot, bit of catchlight in the eye..
As mentioned - the 5D Classic / Mk I is a great camera and on the second hand market a good price - A 5D Mark III will likely depreciate in a year more than just buying a 5D Classic!
You've got two lovely lenses to try it with too - 24-105mm & 70-200mm f2.8l II. You'll lose the further "reach" of the 70-200 but gain a shallow dof of the shots.
Another option is just to line up a few things to shoot in a weekend, and hire a 5DMkII.
As a wedding assistant do you get to use some of the primary photographer's equipment or is it all BYO?