What a fantastic patch of land! It looks very wild and not very field-like at the moment.
Pigs dig everything up so maybe they'd be useful. Otherwise, cut everything down, mow/top brutally and keep doing so until you either kill all the weeds off, or the grass that remains (if any) has a chance to get a good foothold. I would expect re-sowing it may well be necessary. Possibly top sowing rather than ploughing and starting from scratch.
I'd write to all 4 of the owners individually and see what happens. Possibly follow up with a phonecall or visit. I imagine they will price it at whatever they see fit, from absurdly high because of it's useful position to you, to nothing at all because it needs clearing up, especially if there are injurous weeds present.
Good luckwith an exciting project!