The L has no viewfinder at all and no ability to focus (it has no built-in rangefinder -- or viewfinder).
The R has a rangefinder and three user-selectable framelines: 35-50-75.
The L and R are screw mount. You cannot use M mount lenses.
The T has only a rangefinder but no viewfinder. You will need to use/purchase a separate viewfinder for composing your shot.
The R2 is like the R, except it is M mount and can take screw-mount (via an adapter) or M-mount lenses.
The R2A, R3 and every model after that are all M mount.
The use of a 20mm lens on any rangefinder camera will require a separate viewfinder, regardless of model or manufacturer.
If it were me, I would find an R2 lens and couple it with one of the Cosina Voigtlander Color-Skopar 35mm lenses, such as the Pancake II model. Unless you're a high-speed freak and demand a fast lens.