focal length is pretty hard to determine if you don't know. i would say that it is a pretty neutral lens as there is no visible distortion towards the edges of the plate. (plate or backplate is an industry term for the image plane/sequence)
If you think those perspective lines are good then I would use those to help position your camera. you can make a poly plane and translate it back and forth in z or x (whichever heads towards the horizon.) if the plane does not really pass through the plate where you imagine the horizon to be then the camera is not at the right rotation.
use the lines of the grid or make a poly plane help you judge the perspective. if you line up edges along the lines in the grass that might help you determine the focal length.

assumption is the way to go to be honest. I do it all the time at work, if i dont have any camera data... its the only thing you can do i reckon.