J Wal If I were looking for a truly heavy duty rear cutter - I'd be looking at Brown Mfg or Bush Hog. From what I've read on multiple threads - with proper maintenance the Brown 4 series or the Bush Hog BH20 series should last a lifetime. Your tractor is not big enough for the Brown 6 or Bush Hog 320 series.
Whatever you decide spring for the front/rear shields. I ran one cutter with no shields, it’s amazing how far they can throw things out the rear and no matter how low you make the front it will cover the rear of your machine with material and can also throw stuff that direction. It’s amazing how hard it can be to find them after the fact. Good luck.
Why do they put such flimsy tail wheel bracket on them now days? I've got an old Ford cutter that has a lot of iron back there and I'm glad of it. It could serve as a truck bumper. I'll occasionally break off a 2-3" Mesquite branch backing up under them to mow. Those little angle iron brackets wouldn't last five minutes.
Go with a slip clutch. Shear pins on brush cutter will be nothing but aggravation.
I got a VERY used Bush Hog BH286 and I'm fairly happy with it. Around here it's impossible to find anything like it, used. 6-footer. Was [ab]used in a business; previous owner had to weld 1/4" plate steel on it as the top of the deck has started to get pretty well beat. It's an older unit and even at that the tail wheel wasn't all that robust; I ended up breaking the bracket and having it welded (I don't have a welder) and reinforced so it's vastly better than stock. $800 purchase price, $150 weld job and $300 for a new PTO shaft (with slip clutch) and it works as good as a $3k cutter.
Side and deck thickness and gearbox rating pretty much govern the duty level. In general, the heavier the unit the heavier duty it is: the weight, of course, is a direct correlation to steel thickness. My BH286 was about 960 lbs, but with the extra plate steel it's well over 1k lbs. Gearbox is rated 110hp. And they only rated it at 2" cutting capacity!
I really appreciate having a cab when I'm running this cutter through some heavy brush. I've opened up a window to find out that the noise is really intense. Heavier duty cutter means you can make a LOT more noise!
I have the same tractor as the OP and have been running a Woods 6 ft slip clutch with great success since buying it new in 2004. I mostly use if for fields, sometimes tall and thick grass and weeds, with no more than 1 inch brush and saplings...and it is wide enough to cover my tracks, never bogs down. If I had to replace it, I'd look at Woods again first, and compare EA and Bush Hog.