Hi,
You've some interesting choices ahead of you.
In your shoes I'd go for one of the Industars as they only came in the 39mm threaded version,AFAIK. The f/2.8 one is the best in most people's opinion and it will be 50mm, meaning you can stick with the Zorki 6 without an extra 35mm view-finder on top of it. There's also some decent lens hoods for i from cheapo (pennies) USSR made rectangular plastic ones to also cheapo (coffee money) Chinese onces based on the Leica ones.
The Kiev/Contax fit J-12 is a nice lens, if you are getting the bug and are happy with its condition, then why not put it to one side and look for a Kiev, extra 35mm view-finder and so on? You can do this at leisure, which is the best way to avoid mistakes. And there's lots about these pleasant cameras out there on the www.
Regards, David
PS If you've not got a rear lens cap for the J-12 then use the plastic tub from a cassette of Fuji film. That preserved my one many years ago and I've been recommending it ever since.