Good to know about the Contax Donald. I've heard something about the pre-war contaxes being compatible with the Russian lenses in general be default, so it may be a good purchase if you plan on using Russian glass? (Might want to check, I think we ran across that comment on a previous thread).
You might think about buying a broken Jupiter 12 and swapping out the rear element. I've also heard you can polish out scratches using some sort of glass polish and maybe a dremel tool, but that might mess up the focus if it changes the thickness of the rear element.
The only incompatibility I know of between the Kiev 2, 3, and 4 models and Contax II, III, IIa, and IIIa and their respective lenses is that the Jupiter 12 and pre-War 35/2.8 Biogon must not be mounted on the Contax IIa/IIIa (and any other Contax-mount RF camera should be checked carefully) because of the thicker shutter. The first Kiev 2 and 3 models were built on the same actual Zeiss equipment that was used to build the pre-War Contax II and III -- the entire factory was transported to the Ukraine after the War. Kiev 4 was a slightly simplified camera, to be easier to manufacture, and incorporated some of the same changes that went into the Contax IIa/IIIa models -- flash sync, no folding foot around the tripod socket, etc.
Where compatibility issues arise is with Nikon's (almost) Contax mount RF cameras. These are perfectly compatible with Contax/Kiev lenses that use the internal bayonet (50mm only, AFAIK) and within tolerance on outside bayonet lenses shorter than 50mm, but won't focus correctly with longer Contax and Kiev lenses (which all use the external bayonet).
In fact, I did buy a partly disassembled LTM Jupiter 12 (optically identical) for $10 plus modest (slow) shipping, I just haven't gotten around to swapping the rear groups and testing the Contax-mount lens for optical quality. This became less urgent, as before the junker lens (with good glass) arrived I bought a second Contax-mount Jupiter 12 from a Photrio member and I've been quite pleased with it. I don't use wide angle as much as I do normal, and the Jupiter 12 has that idiosyncratic inside aperture adjustment, which makes it, um, interesting to use with filters.
I probably should get around to doing the swap and test so I can sell off the repaired Contax-mount lens, or explore what would be involved to transfer the entire optical block from the presumed damaged LTM (Fed, Zorki) focus unit it's partly removed from into the good Contax (Kiev 2,3,4) focus unit.