I have a FED 3 and Case for sale. Lens is 2.8, 55. Rangefinder is accurate, shutter speeds are functioning and sound accurate. S/N is 6773352 and includes take up reel.
The FSU lenses have some slight differences and sometimes, not always have to be collimated with small shims made of aluminum foil (.05mm or so)to make them focus correctly on a Leica range finder body. If you stick to FSU bodies and lenses though you theoretically should be fine.