I have a sale pending on the Kiev and lens. Contax and Kiev are fully compatible. Compatibility between the Nikon and Contax mount is a murky subject, but as I understand it for wide angle lenses the focus errors are small with respect to the depth of field. It also depends on whether one is using Nikon on Contax, or Contax on Nikon and there are even lens specific incompatibilities. Longer lenses require significant focus compensation (best to avoid them). I think 50mm is about as the longest one would mix the two lens systems and this may require scale-focus corrections at wider f-stops.
I have heard of Contax Sonnars being used on Nikon RF bodies, but pay attention to the focus corrections (there are Web sites and forums that discuss this at some length). The Sonnar and the J-8 are quite similar so that implies the J8 would probably be OK.
By the way, probably because they made so many of them, the Nikon 50mm F2 and F1.4 Nikkors for Nikon RF are not too expensive and are exceedingly good lenses - amazing really. Most of the other Nikkors are pricey, sometimes extremely so.