As for cutting 15 acres and size tractor / mower needed. My retired brother in law cut our combined ownership of 42 acres of heavily overgrown pasture and at the time, weeds and small saplings with a 750 John Deere (19 PTO HP)and a 4 foot bush hog. I think it took him about a week but he didnt have anything else to do and besides, it was too rough to go fast anyway so a more powerful tractor would not have helped in speed but of course more width is always good for time reduction. It is really easy now to go way overboard (this from one person who has been there) with needed HP when you look at tractors and capacity. Myself, I have way more tractor than I need for mowing, grading roads etc but I have to say that sometimes even 70HP is not enough like when I was trying to pull out a stuck dump truck loading with gravel but I did eventually get him out.
My Dad farmed 40 acres with a then new 8N Ford which was 1000% better than a mule team so it can be done. Man how far have we come in one generation.