Just in case the idea of "coupling" is unclear...
With an external meter or a clip-on meter such as the VC, you need to take your reading, and looking where the meter needle is on the scale, transfer both the recommended aperture and shutter speed onto the lens. With a coupled meter such as the Leicameter, you look where the meter needle is on the scale and transfer that aperture onto the lens. The meter already knows what the shutter speed is and will give you the correct aperture; that's the advantage of a "coupled" meter. (The price you pay for this added convenience is size; the meter is slightly larger than the VC non-coupled meter). Another advantage is that you can take your reading, then recompose, and the readout will still be accurate for the part of the scene that you metered. Having access to the instruction book is a real help for getting successful results.
As with any electronic gadget, generally the newer the better; the latest model of the Leica meter is the MR-4 which was issued to match with the (then) newly introduced M4. The latest iteration of the MR-4 is the black one which matched the black M4 which I'm guessing was introduced somewhere around 1970. Like the Leica itself, it's a sturdy and heavy (for its size) unit, and if not abused should function as smoothly and accurately as any other contemporaneous light meter. If you're lucky enough to score one that was only owned by one previous owner, and comes complete with the instruction manual, it will give you good results with any kind of film. Just be sure to use a compatible battery; Ken Rockwell's site tells you where to get a modern battery that will give voltage similar to that of the no-longer-available mercury cell (just enter "Wein" into the search box on Ken's site; that's the name of the battery). The use of inappropriate batteries with the MR meter may have helped give it a reputation for not being accurate: wrong battery voltage = wrong reading.
