I disagree with much of the advice given here. The most direct path to your goals, it seems to me, would be the 5D Mk III plus a simple adapter for your Nikon glass.By that same token, why wouldnt he just get a 5DII with that same adaptor, save $1500, and use that $1500 to complete his lens collection. An 85mm Nikon prime and probably something else on the wide end and he'd be better off than a 5dIII (which is more of a stills update than a video update) and no extra lenses.
That said, I agree with the others. If you have the money, get a dedicated video camera. There are adapters for the FS-100 and AF-100. With them, you can shoot uncompressed, which will improve your resolution and your workflow.
5D2 = no 50fps, worse rolling shutter.
Use the $1500 for 2nd body (used 7D) and lens
DSLR vid is also really turned into a culture of its own.
You really have the fanatics, pairing 5D2 w/ almost anything. The grain structure on 7D can be pushed to look like 16mm - again if that is your thing. Finished production last year where 5D2 was b-cam (flyovers mostly) for Arri4 35mm film. All worked out in post.
I like the advice on waiting through NAB. Try to rent or borrow. Anything beyond the $6k mark is bought mostly by productions and rental houses. Although you do find $58k epics and $80k alexas in private hands - its just not that common.