Has the rangefinder got to be coupled? Or are you prepared to find the range then adjust the focus?
Must you have interchangeable lenses? Or is sticking with one focal length acceptable to you?
Are you happy to use an old camera from the 1950's? Or must it be from the 1980's onwards?
Are you happy to carry a large camera to get the advantages of 120 film? Or must it fold away to a small size?
How much are you happy to spend?