It depends on the type of soil, size and condition of the plow, ground cover and your reason for using it.
Many 2WD Ford 9N's pulled a 2 bottom plow and were approx 23 HP. Your 4WD would probably do as well. The smaller plows have less resistance and would do a bit better than the larger sizes. Plows should have no rust (a rare thing to find these days) to work well, so if you are looking to buy an older one, ensure there are at least no pits in the working surfaces and is capable of taking a nice shine. Otherwise, it will not be self scouring. If you are buying new, always clean and grease plows before storage. Do not leave plows out in the weather - store inside the barn, up on blocks
Heavy, dense soils will challenge a decent pull, lighter loams will not pose much problem. If you plan to plow in heavy residuals (needing a coulter to cut them) it will also make it tougher. Rock is another story.
If you need to break new ground or are just simply breaking up soil, it also makes a difference. Also bear in mind that a plow can create hardpan in heavy soils.