Along the Leicas, I use some FED NKVD's from 1939 and one Zorki Ic (1952).
If I had no camera and wanted to buy an FSU RF, then I would go for any FED I (up to If) - Its the closest to the original Leica II and most of the time well made camera.
Any camera from that vintage might need some CLA and patience but once its all set - You will love it
Some shots from my gallery were taken with FED NKVD with its original, matched uncoated 50mm f/3,5 lens.
In the USA, http://fedka.com/catalog/ looks like the best place to go for those beauties.
There might be some other reliable sources for those cameras as well..