George, the J-12 is an excellent lens in the Kiev mount. I loved mine.
That said, buy the CV Color Skopar SC 35/2.5 instead. I got one of those and never used my Jupiter again. It's a utterly astonishing lens - as any modern lens with modern coatings will be I suppose. For the very wides, the CV's are the only ones worth buying - the few FSU options are overpriced and underperforming. The Zeiss lenses perform well but are _way_ overpriced.
Normals - I liked the Helios as it has a signature that is actually distinct from the Jupiter lenses. The Soviets did a decent job on the design and it's well worth it. OTOH, the 50mm Sonnars are what made the Contax a world class camera system. Find at least one of them - CZJ 50/2 or 50/1.5 or Jupiter 3 50/1.5 or Jupiter 8 50/2.0 - so that you can see the difference. That difference is especially noticable if you get a CZJ 50/1.5 pre-coating... 😱
However, for long lenses, the Jupiter 9 85/2 & Jupiter 11 135/4 are easily the equal of the Zeiss lenses. Get the cheap copies - no one will ever see one whit of difference unless you tell them.
Gack. I really need to get myself another Kiev... maybe a nice cheap 4AM with a j-8... 😉
HTH,
William