Is lens quality on the 5d ii as crucial for HD video as it is for still photographs?
Please forgive me as I have the 5D mk1, with no video
That said, I've used all the major motion picture lenses and many higher end video lenses. I will say that the optical quality of good Canon lenses will be more than adequate for motion capture on a 5d II. Many movies have used Canon and Nikon long lenses converted to motion picture camera mounts. Even Panavision offers these.
The real issue here is keeping moving action in focus with very, very shallow depth of focus. Auto focus lenses are not designed for this and you might just find yourself tearing out some (or all) of your hair
The big advantage of motion picture lenses is that they are manual focus, have accurate distance scales, and a long rotation to place enough space between the footage marks. As you may, or may not know, motion picture focus is usually done by a specialist who's job it is too estimate the distance to the moving subjects and adjust the lens accordingly.
I think I've heard that Zeiss is planning to build a special version of their manual focus primes with a long rotation for use shooting motion on the 5dII or 7d. If this is true, then these may very well be the best choice of lens to use with a 5d. Short of that, perhaps some old manual focus nikon or pentax lenses and an adapter will be helpful, but the lens rotation is short and that will be a challenge.
There is no real way to view critical focus yet while shooting with the 5dII in movie mode, so focusing by distance with moving subjects is really important.