Stephen
There is not much functional difference between the C and K, the early K are pretty similar, and you should he able to get a 'user' Contax body resonably cheap while you wait, for HS.
The only initial differences are in the finish, reduction of parts (screws) and with (some) later lenses the black paint. The FSU rationalised the design, e.g. shutter ribbon, and added flash synch. The FSU chrome is harder wearing if not as shiny.
If you wanted to chase after your leak you could load a chrome film, and case and tape off leaky points, in succession to try and identify the source. The chrome film allows the side of film with the leak to be more easily identified unless you can do complementaries (colour) in your head.
You also need to look at your J12 unless you are sure that the leak occurs with other lens as the j12 flare can appear similar to a leak. If the J12 has an untreated rear element bevel of ground glass you need to paint this carefully with two coats of black board paint first. My most annoying leak dissappeared after this. I rebuilt the camera first, good fun if you like that sort of thing.
You may need to replace the internal black paper as Russ Pinchbeck describes, etc.
What sort of leak do you have, what is its colour, and does it entend into rebates?
The rangefinder should be really solid unless it needs a rebuild...