I would just plow and disc to cut the dirt up and then rent a harley rake to go behind your tractor. The harley is depth adjustable and will angle to either side to throw out any excess dirt as it cuts down the humps and fills in the holes. If you start on one side of the pasture and work across to the other side, then go a second pass going from end to end, you will have golf course smooth pasture. Total cost will just be the rental on the rake and your fuel, and of course your time. Added bonus of using the harley rake is it will rake out all the rocks and debris into winrows for easy scoopup with your FEL. With just 15 acres, I bet you could be done and ready to plant grass in less than a day.