How rough is the field? First off, I wouldn't reccomend clearing brush with anything other than a brushhog/rotary cutter. It can be hard on the cutter, and the BH is built to handle that. Once the vines and brush are cleared, a fairly regular mowing will promote grass with the occasional broadleaf control. Once the heavy stuff is done, a little touchup to the blades(1/8" cutting edge) and a brushhog can give a suprisingly good cut on grass, as long as it is run at it's full RPM. If it is used like a finish mower with mowings frequent enough before the grass gets too tall, the clippings will mulch in evenly. If done less frequently, the taller grass clippings clump and smother the grass giving a real rough looking area, bare patches that fill in with weeds ect... It is basically just a real big single blade lawnmower with comparable tip speeds(13000-14000 FPM) which is what does the cutting. The problem is that a big 4' square(or larger) mower deck on a 3PH dosn't follow undulating terrain as well as a 2' X 5' finish mower would. I have 7 1/2 acres about half in timber. Still "parking" quite a bit of it which the brushhog is good at, but the cleared areas that are in grass now, I also mow with the hog and it does a fair job, faster with less fuel burnt than my 48" rider will...
A flail mower would follow terrain well as it is not very long, but if you hit something, or when worn, sharpening the flails is a LOT of work, and replacing a set costs a small fortune... For cut width, I don't think they really take any more HP...
