I used to mow 6 acres of grass pasture on a slope twice a year with my New Holland Workmaster 55 (55 HP) and Woods 5’ rotary cutter and it took me about 4 hours. I had a 39 HP tractor before that with the same cutter would take me 6-8 hours depending on the thickness and how hot it was out (the tractor would get hot if I pushed it too hard). I wish I had a larger mower when I had the NH tractor as I could have probably done it quicker.
I agree with the previous question do you need to trailer it, as that will have a big impact on the size you choose. Also consider how much time you want to spend mowing it. If you don’t need the tractor for anything else something simple and inexpensive will be fine. However if you have other potential uses you could start considering the benefit of paying more and getting features like a loader.
So, do you need to trailer it, how long do you want to spend cutting it, and is the property on the flat or hilly?