Based on your list, a MF 265 or similar will do what you listed, for around $6-10k. Moving more modern, and more expensive, something like JD 5205 would probably be between $12-20k. The other route, something like a TYM 574, brand new, with loader is like $33k. Set aside about $1600 for a 6-7 ft bushhog;
For 40-80 acres, you might want to explore a 9-10 ft bush hog, and that would probably push you towards around a 70 hp machine. I'm assuming we are talking about at Most mowing every two months, during growing season, maybe once between frost and spring; so probably around 4 times per year; a 6 ft mower, even at 5 mph will take a good long while. A 8-10 ft bush hog used is going to be $3500+; and new. probably more like $8500+.
In a perfect world; 6 ft cutter; 5 mph; is 3.6 acres/hour; a 9 ft mower is 5.5 acres/hour at 5 mph. You could maybe get it all cut in a weekend.
Edit: 80 acres, depending on condition, might be worth renting out to a farmer, if the terrain, soils, local economics make sense; might bring in enough to pay property taxes, keep it ag exempt, and put a Little in your pocket.