I run a 6' easily on a '65 Ford 3000D and bought 6' to cover my tractor's tracks. You can www Tractor Data for stats on it.
I bought one several years ago, my first. Looked at a lot of them and in general they are all sturdily built, seem to all use the same gearbox with a made in China tag on it, just different companies do their bracing differently. I have 54 tynes which is common and seems to be the HD model.
TSC had their Bolens produced unit which is what I bought. I put it through dry, heavy clay and it made a lot of noise but pulverized the soil. HP is not an issue. Could probably run it with 20 and have some to spare....but it is like 700#...heavy for a little tractor but the 3000 with no weights did ok with it, no front end bobbling or loose steering.
The ground speed and tail gate height you set determines how fine a job you do. The slower and lower the finer. For a fast weed to soil incorporation they work great. On stemmy grasses, I'd brush hog first to chop them up into little pieces so you don't have stems wrapping around the main axle.
TSC has them on the Labor day sale right now.