There are many posts on essentially the same topic. I was looking for running a 6' cutter and moving round bales. I wound up with a JD 5105, which will run the cutter with ease, and while I haven't tried yet should also move round bales with ease. In hindsight I didn't need quite as big a tractor for these tasks, though it gives me a big margin of safety and ease of use.
Based on my experience so far, I think 5hp per foot of cutter is a good estimate of what you need, but if you can afford a 5-15hp margin above that, you will probably be happy to have it. I believe that Deere indicates 40hp minimum for a 8' pull type cutter and 60 for a lift type cutter. There is no reason lifting the cutter actually takes more hp, I believe the number is just to preclude people from hooking up a 4720 to an 8' lift cutter and expecting to have a useable setup.
For these tasks you would probably be ok with the largest CUTs, at least for a 7' cutter, or you would be in comfortable territory with most utility type tractors (JD5105 or 5225, Kubota M5040, New Holland TC55 and TN60, etc.). If you want the 8' cutter you will probably be happier with a utility tractor, though a large CUT may work ok with a pull-type 8' cutter.