Let me say I know the Mark 2 is better in every way then the Mark1 but I have to scrape together money for the 6D body let alone a 2K lens. So I'd debating weather I buy the 6D kit or just body and a 24-70mk 1Get the kit. Its a bargain. The Mark I is overpriced, and used 24-70mm L MK I lenses are nortorious for problems. Roger Cicala of lens rentals has found some apparent reasons for problems with 24-70mmL lenses with high usage.
If it helps people answer... I don't' pixel peep or anything crazy, I'm in the group that if 90% of the population who are not photogs think it looks good then that's all I need.
I have had several used 24-70mm Mk I lenses that were not very impressive, they just did not seem to be as good as they should be.
Personally, paying a very high price for a used one might lead to grief. If its been inspected / repaired by Canon, its likely OK. Lens Rentals does do a resolution test on their used lenses before they sell them, so thats a fairly safe bet.