Usually the manufacturer of 3 point PTO devices recommend the minimum HP for the device in their specifications sheet. I am thinking you could just google the brand name and locate the spec sheet online. That might help take a lot of the guess work out of which tractor to match it up with. For instance I found that a six foot King Kutter recommends 35-50 hp. I am thinking that is PTO hp, so engine hp would be a little higher.
In the past I have set my tiller depth to one or two inches to till over a grassy area that is to become a garden. I then skim the grass off with the FEL. If the soil is a sandy loam mix or has been a garden before I would increase the depth and till. If the soil is clay, dry and tough I try to plow it first then till. Seems to be much easier on the equipment that way.