I used to have the 19mm R lens for the R system. In general, it is a great lens. I find that using adapted lenses generally yields sub-par results in at least one aspect, sometimes many. The usability and speed of use go down, often the adapted lens performs worse on the new body than it did on its own mount (vignetting, poor results because of bad interactions with the newer body's cover glass or sensor design etc). At least in my experience, any performance gains (which were usually negated by poor lens/sensor interaction) were offset by the clunkiness of the solution.
My suggestion would be to invest in the upcoming Zeiss 16-35mm zoom. It will likely perform similarly well on the A7r as a 19mm R would, and it will be more integrated into the system.
Not to be too brutal, but if you want to enjoy Leica lenses, you should really use them on a Leica body. That's just my take after struggling with adapted R lenses on a D3, which actually adapts fairly easily and with M lenses on micro 4/3rds. I know a lot of people here don't feel the same way.
P.S. I don't know about the ZE/ZF distagon, but I found that the ZM 18mm f/4 Distagon performed better than the 19mm Elmarit R, at least to my eyes. It had lower distortion and better edge to edge performance. It is a bit of apples and oranges though, as one was an SLR lens and another a rangefinder lens. I would suspect the R 19 would perform better on the A7r due to the angle of incidence issues with rangefinder lenses on digital.
