I've got a B+W MRC UV on mine to protect that big and beautiful front element.
If you plan to shoot outdoors wide open (portraits, etc.), you might also consider a 3-stop ND - sometimes even 1/8000 s isn't enough.
There will likely be no difference in AF speed on the 5DIII vs. 5DII. I do notice it focuses faster on my 1D X, but that's because the 1-series bodies use a higher voltage battery which drives the AF motor faster.