Start by being more specific about the model of B8200 you have. Particularly the transmission. I assume it has 4WD.
There is a free Owners' manual on Kubotabooks.com for a manual trans model but many of the other details will be similar if your has a HST.
Manual
Your intentions are not different than most new owners of these small tractors. They are impressed by the tractors capabilities and start thinking they have bought a full sized backhoe or skid steer.
On a good day you have 16 HP at the pto. The engine HP number of 19 is not useful for you.
Depending upon the model you have the hydraulic flow is at best 6 gpm at full engine speed.
The flail mower you are looking at requires a minimum of 13 gpm and better performance at 21 gpm.
Since your existing hydraulic pump does not meet the requirements of the flail you decide to buy a pto driven pump and reservoir. To produce 13 gpm at 2,500 psi would require 19 hp. In other words the engine of your tractor does not have the power to drive a big enough pump to run your flail.
An alternative is to drive your flail with its own gas engine using the loader to position the flail.
You have a machine many would love to have. Respect its limitations and it will serve you for many years.
Dave