I can only comment specifically on machines produced in the antipodes but from my perspective a topper or 'slasher' would be an appropriate machine. They're much like a large rotor-cut mower (ie. the disc rotates around the vertical axis) but dragged behind, and driven by PTO from, a tractor. Sorry if that sounds a bit obvious, not trying to teach anyone about sucking eggs, just want to be clear.
Typically (where I come from) these things are rated in hp - or at least the gearbox is - and there are two predominant types of blades. Probably the best for you would be the type that has smaller blades (say 4" - 6" mm long) and the blades are attached to a disc (which in fact may be a square flat plate, just to confuse!).
Others units can attach bigger blades to single (or occasionally multiple) bars - these machines often rotate more slowly, probably something that wouldn't work so well in your environment but ok when you're trying to work through some heavy country (think lots of gorse).
The disc-type are generally adjustable in cutting height and usually run on skids. In my view a single-rotor machine is probably more reliable and less expensive to run, with the downside being that you may not get as great a cutting width and so need a few more passes than a multi-rotor. Multi-rotors may split the driving power via belts or driveshafts and/or gearboxes which requires more power, and obviously have more moving parts to break or wear out.
I have several such machines and the cut would be fine (and has been used
for an airstrip. I have a smaller '50hp' unit that I use with a Ford 4000, and a much larger 180hp machine that's used on the bigger place with a 100hp tractor. The former produces the best cut from an airstrip perspective but it's probably around a 4 or 5ft cut whereas the larger machine is around 6ft or more I think - they're not handy to where I am to measure, sorry. The multi-rotor unit(s) died and have not been repaired but did an ok job when operating - to be clear these are
not hay mowers, something else again (although most are multi-rotor disc cutters).
A link, if Pprune will allow, to some machines along these lines is
here,
here,
or here. I realise they're way too far away to think of, but it may assist in giving an idea of the sort of thing we consider to be generally robust and something that I think would do a good job for a long time.
FP.