Look at the mowing calculator Jeff provided and figure out how much tractor you need compared to the time you want to spend mowing and doing other chores . I find mowing very relaxing as I am retired . With about 52HP at the pto I can mow some pretty heavy hay in fourth gear on level ground with a 6ft bush hog . That translates into about 3 to 3 1/2 acres an hour or about 2 acres in 3rd. for me . Some of my fields have gotten a little rough over the years so 3rd. gear is used in those areas . I could not pull a larger cutter in 4th gear for sure , unless it was a dual spindle which I have never tried . It's much about the comfort , time , tractor , dollars you are willing to spend , and of course compromises .
3rd. =2.4 mph
4th. = 3.6 mph on my tractor