I found home one of these
NIKKOR-S Auto 1:1.4 f=50mm No.348204 (http://www.destoutz.ch/lens_50mm_f1.4_348204.html)

what's the correct adaptor to buy? any risk of damaging the 5D?

It's probably OK but there are some variants of Nikon lenses that have a metal flange protruding that would foul the mirror on a 5DII. Here are a couple of posts discussing the issue with photographs so you can check whether your lens has the flange.
http://www.dvinfo.net/forum/canon-eos-5d-mk-ii-hd/146809-nikon-lens-adaptor-5dii-very-very-tight-fit-normal.html
http://www.dvinfo.net/forum/canon-eos-5d-mk-ii-hd/469743-will-non-ai-nikon-lens-will-work-canon-5dmk2.html

It's a long while since I last bought an adaptor but I just got the cheapest (about £10) passive adaptor that I could find on eBay & it worked OK. There are now slightly more expensive (about £20) adaptors that have a chip on board that gives AF confirmation.

I have three separate Nikons in that style that I use regularly, the 24mm, 35mm, and the 50 mm. All work fine, with a very inexpensive Kawa adapter. I know I had to peel of a protruding flange on one lens I was working with because it interfered with the mirror operation, but don't recall if it was on the 50mm. Very minor and it does not affect lens at all. I think purpose of the flange is as a rest for a person that might lay is without rear cap on flat surface. You can see a flange at that the 6 oclock position of the rear shot of the lens at the link you provided.

I got the adaptor and everything works fine.
shooting with a 1.4 lens is great!

can I get some clarifications to the shooting modes with a manual lens?
if I go in Manual mode, I cannot seem to change the aperture: it's essentially stuck at whatever value (say 50) I left it when I last used the 24-105 lens. is it normal or should I still be able to change the opening times?

also when I go Av the aperture doesn't change (which is what I want) but remains set at 50, no matter what - I cannot seem to set it to a different value. I cannot seem to do Aperture priority with a different Aperture than 50: again, is this normal?

finally I guess the only way to shoot with a faster time (say 125) would be to use Tv and then lock the AE using the "*" button after maybe using a fast shutter value and pointing at a light to get the camera do reduce times to compensate, to the desired value?

thanks
Fed

Here's what I do:

When shooting with a manual lens on your 5Dmk2 --the aperture ring on the lens determines your aperture.

Shoot in the "M" mode[Fully manual]

The front wheel by default now determines your shutter speed--I leave mine set at 1/50 always because of the 180 degree rule so I get the nice filmic blur.

Make sure your ISO is not set to auto. Tap the ISO button once on the top, then use front selector wheel to change your ISO.

So when shooting video:

Chose the aperture you want on your lens.

Now adjust your ISO for proper exposure using the light meter.[Tap the ISO button-- then adjust the ISO].

Not forgetting that if it's too bright you will need to put a neutral density filter on the lens to get the exposure right (use a variable ND filter for convenience).