A Ferguson TO30 won't have any trouble at all slinging a 5' brush hog (rotary cutter). Likes been said you definitely need a PTO ORC (over running clutch) adapter.
Basically stated, the PTO on these old Fergies is driven directly off of the transmission. Meaning when you engage the PTO via the PTO lever, it is directly connected to the tranny and rear, which is fine until you want to stop. Normally when you depress the clutch this disengages the engine from the tranny/rear/PTO and you'll stop pretty quickly but, if your driving an implement off of the pto that has alot of momentum due to it's swinging mass (such as a brush hog), this momentum will continue to drive the rearend, hense the wheels, since they are connected together. This has driven many a poor fellow through fences etc. What an ORC does is allow the pto driven implement to just free-wheel after you depress the clutch, and this effectively disengages it from the drive train. The ORC simply slides over the pto output shaft and is pinned there. works good lasts a long time
My old TE20 had no problem at all swinging a 5' medium duty Howse rotary cutter. It would spin a heavy duty had I had one. For 16 yrs it keep my fields knocked down as well as plowed and disc'ed my garden. A pond scoop acted like a 3ph mounted loader and a boompole on the 3ph would lift over 500 lbs.
Things to look for:
1) The radiator, if theres no leaks, it'll like to run with the coolant about 1 - 1.5" below the fill hole. If you fill it higher it'll just shoot it out the blowby until it hits that level then it's happy. The thermostat mounts in the upper radiator hose and I highly recommend installing one if it doesn't have one.
2)The OEM rear axle seals are notorious for leaking the hydraulic fluid out into the brakes making them useless. If you see grease trails on the back side of the brakes the fix is fairly simple. Pull the wheels, brake hub, brakeshoes, backing plate and axle then install an after market "sure-seal" into the housing then reinstall in reverse order. Replace the pads and now you've got very functional steering brakes and no more leaks.
As has also been pointed out, you'll need to install a couple check chains from the top link housing down to the front of the rotary cutter, these act as your depth adjustment for the cutter. The chains, and small brackets to secure the chains to the toplink housing at the pin can be had at most tractor supply's, CoOp's as well as via some web sites targeted for Fergies. Just google Ferguson tractors. Manuals and parts are still very available for these machines.
Remember that the 3PH lift will not operate unless you engage the pto. Meaning if you want to lift something with your 3ph, the pto will also be spinning. This is fine unless you have a pto driven implement connected. To keep from having the implement spinning when all you want to so is just lift it for transporting etc, you'll need to disconnect the implements drive shaft from the pto. This is very easy and will become 2nd nature after awhile. What I did was make up a large loop made of chicken wire and draped it around the top link. When I disconnected the driveshaft from the pto I'd then loop the driveshaft through this loop which would then hold it up and out of the way of both the ground and spinning pto.
The more I write this the more I miss that old hard worker.......
Good luck