Hi Anthony, FD 20-35 f/3.5 is sharper in the covering range and it is very nice landscape lens at apertures arround f/8-f/11. But it is usually more expensive. (if in really good condition).
Shuter shock on A7r is a tricky issue, hard to predict when and where it will occur.
I am keeping my fingers crossed for your search of Canon FD 300/4L for a reasonable price.
Cheers,
Viktor