One pointer, the cheaper toppers (like the fleming we have) rely on shear bolts to protect the topper from damage when you hit a stone. If your land is stony, you will get through industrial quantities of shear bolts. More expensive ones are more robust but they have more bits to go wrong.
For our land, we have had much more success with an ancient flail mower (no identifiable make sadly), with rows of curved metal blades mounted at right angles onto on a pole/axle which is horizonal to the ground inside a metal casing. Unlike a topper where the single long blade itself is like a lawnmower, horizonal to the ground just spinning and therefore much more prone to damage.
So our Fleming topper is fine for the nice grass but for ruffty tuffty fields, flail mower every time. Ours is 10 foot and hasnt killed the 75hp old John Deere yet! I wouldnt go any bigger than that tho. 7 foot might be ideal, I would go for 7/8 foot rather than 5 or 6 as it makes a lot of difference in how long it takes to cover a field and also how much fuel it takes.