I had a JD 530 which rolled a 5x6 that I purchased from an auction co. for $5k about 8 years ago. Pulled it with a 100 pto hp tractor and wouldn't go below 70 hp with it I don't think. It would tell you when the bale was getting full. Bale weight was 1800 to 2000 guestimate.
Currently have a 375 that I bought from the same co. last year and rolls a 5x4 for $5k also. Easily pull this with a 57 pto tractor. Maybe a 45 or so would work ok. The 57 isn't working hard. Bale weight is about 800 guestimate.
Both balers were in good shape. The 530 required more repairs.
Other thing was my land is reasonably flat. Don't know the effect of hilly terrain on tractor size limitations.