Will you use a heavy disk, duals and extra ballast for your 4020 very often? For only 50 acres, if you won't the stuff again in the next five years why buy all that to have it sit in the shed and deteriorate? Maybe rent a smaller heavy disk that you can pull with your tractor as-is, hire someone to do the work, or use a smaller molboard plow, they are cheaper than a disk.
In most soils a 4020 would pull a 5 bottom plow or a 4 bottom plow in a higher gear and get about the same amount of work done either way. Take it easy on the old girl and pull a smaller implement in a faster gear.