I cut an acre of grass also....
6.5 hp Craftsman mower - 6 hours
16 hp Craftsman lawn tractor - 3 hours
21 hp Craftsman lawn tractor - 1 hour
The 21 hp had two cylinders, thus it never bogs. 6" of grass is no problem at high cutting speed.
The 16 hp will struggle in deep grass, half cutting speed must be used. If there are hills then it really heats up.
OPINION
1)16 hp minimum
2)two cyl is best
3)Craftsman products as the manual and parts can be accessed on the internet.
4)Cast front axle , now you can push Snow!
