We used those a lot for waterways, terraces, etc. many years ago in south Georgia. Like an FEL, they don't dig into hard dirt very well (if at all), but a fairly small tractor can move a lot of loose dirt and spread it fairly smooth. Seems like we used Massey Ferguson 65 Diesels (50hp?) on dirt pans just a bit larger than that. The issue is not so much power as it is traction. If your tractor can push a FEL with a bucket that wide into loose earth, it should pull the pan as well.
There must be something on the rear crossmember that catches the edge of the back gate when the pan is rotated and lifts it away from the pan so that the material can slide off the back of the pan.
If the hydraulics and cutting edge are okay, there is not much left to worry about. I would probably pay $2,000 since you would spend nearly that much on parts alone to build one.