Of all the R series, the R6 or R6.2 should theoretically be the most reliable because all it has is mechanical shutter speed and aperture; the only electronics are the meter and self timer. No program, aperture, or shutter priority electronics of the R4, R5, R7, R8, or R9 cameras.
The R3, of which I owned an olive drab Safari kit briefly, lived up to its reputation of electronic unreliability.
As net legend has it, R4's up to a certain serial number share similar foibles as the R3.
For full aperture metering, you will need 3-cam lenses for an R6 - not a bad thing, just more expensive.
http://www.photoethnography.com/ClassicCameras/LeicaR6.html
My only reflex Leicas are the R8 and two SL's.