You may also like to consider asking a neigbour with a smaller tractor and topper to clear the land, (assuming you are sure there is not a load of angle iron, fence wire and the like hidden from view)
I cleared a few acres with a Dexta and 6' Amex topper for a neigbour a few years ago and have used it for sheep ever since. I did not use roundup as that would have necessitated re-sowing with that added expense. The first year, in addition to the sheep, I had to mow it a couple of times to prevent brambles, burdock and some thistles re-establishing themselves, but since then, sheep and one topping a year has resulted in a decent acre or two for grazing.
During my initial topping, some of the growth was as high as my head seated on the tractor, and I did proceed cautiously with the topper as high as I could sensibly hold it.
I've justg looked at your posting again, and can see that you do not intend to put animals on the land, but intend to re-sow. In that case I agree with glasshouse to put pigs on for a season. You could try topping to clear the rougher stuff as well as.
If you are thinking of growing veg or something however, spray it prior to planting.