If you are going to do all of the mowing yourself Hp matters only in terms of how big a mower you can use. You need around 30 PTO HP to run a 6-foot brush hog I think. I brush hog about 10 acres witha 5-foot and I would sure not wish to go to a 4-footer. It takes about 7 hours usually, and thats if I don't let it get too thick. 15 acres would be butt-numbing, plus the other 8 you want to finish mow.