I have very little experience with land conversion or farm equipment directly, but I am getting somewhat familiar with things from working with large-scale wheat farmers. I would suggest not worrying too much about a strict on-contour plowing. It's a great idea to implement as much as possible, but not worth messing with picking up the plow to turn it around - I would recommend just turning around with it in the ground. The edges of fields are usually different from the interior anyway.
The harrow in your picture looks like it would be pretty aggressive to me. If you're doing one-time soil prep for the use conversion, I think it would work, but that level of disturbance often probably isn't great.
As an alternative, have you thought about running large numbers of chickens, and lay down straw, manure, etc. for them to pick through and till under? You could do a portion of the field at a time, and after letting it rest for a few weeks to let the weeds sprout, run the chickens back in again. It might take a year or two to get the field cleaned up enough to do what you want with it, but you'd be a long way towards getting your soil conditioned and fertilized.
Those with more experience than I, feel free to contradict anything I've said.
