You should be thinking loader breakout force and lift rather than horsepower for that sort of task. The two (loader and hp) are not always correlated. I did way more brush and tree removal with my old 20hp Kioti CK20 with 1100lb lift than I could have done with a 30hp Kubota
B7800 simply because of the loader was stronger. The Kubota could mow circles around me because of the extra HP but mowing was not my primary concern. Hard to justify more than 25hp or so for one acre though. And spend the savings on a good light duty 48" grapple and a ratchet rake for the bucket. That combo is great for clearing and prepping pasture. In the long run you do not want 40hp and six foot implements on a plot that size. With one acre get a small CUT (not a BX) and spend the extra on a grapple, backhoe and ratchet rake. You'll have a clear field in no time and an appropriately sized tractor to maintain it afterwards.
Here is what a 20hp tractor can do to brush: