Most 45 hp tractors will not have adequate PTO HP for a 12 foot mower. I have a 45 hp tractor. It handles a 7 foot heavy duty pull behind just fine, but it would not handle much more.
As others have said, you really don't need to be thinking about what equipment you need until you know exactly what you want to do and how much of the land you'll be doing it on. If all of the acres are in pasture and you want to do hay, you will need more than 45hp and probably more than one tractor, not to mention expensive implements. If all you need to do is keep the pasture mowed, then a 45 hp tractor with a 6 foot mower will be fine, but it will take all day. I've done it, its not all that fun but its not all that hard either. It is much easier to do all that in rotation rather than all at once. If you need to mow it all at one time, every time, think 55-65 hp with a 10 foot mower.
It all just depends. I have about 250 acres. I'd say less than 20 of it need mowing, including the roads. Forty five hp is all I've needed. Would love to go with something like a Kubota 6040, but just can't justify the need and in the end, my smaller tractor is easier to handle in the woods and on the trails.