The 75 hp is a little low by todays standards but still quite capable. remember with a skidsteer in the situations where you begin to run out of power it is vary easy to slow your ground speed until the engine recovers. The situations where you run short would be ie, running a brusher in thick growth and climbing a steep hill at moderate to fast speeds. Both your hydros (drives) and front quick couplers would be putting a heavy demand on the engine. In this case you would drop your speed to what the engine can manage. Would 100hp be better. Of course, but it will need more fuel, and it will only be better in the situations where you can use high wheel speed.
As for the pressure and flow. Flow is what dictates your brusher rpm, the more flow the less engine rpm needed to run your cutter the necessary speed, or the faster you can spin your cutter at max throttle. Most cutters need to spin in a certain rpm range, so the size of the motor (on the cutter) needs to match the volume of the pump on the machine. remember you can always vary the volume down some by reducing engine rpm. I believe that volumes measurements are taken at full rpm.
Pressure on the other hand is what tries to keep everything spinning. Say you engage the oil to your brusher and it starts to turn. The pressure in the lines will be high until the brusher rpm gets up to the speed that matches the loader engine speed. Then the pressure tapers off and may only be 150 psi. You slam the throttle open and the pressure spikes to max relief valve setting (say 2100psi) again until the cutter catches up, then it tapers off. So you got to work, mowing light brush and the pressure varys between 150 (no drag/load on cutter) to say 1000psi because you are only cutting 3 or 4 sapplings per foot of travel, the momentum (flywheel effect of the rotating parts) of the cutter is whats hacking off the brush and the 1000 psi is enough to bring the cutter rpm back equal to the engine speed in between sapplings.
Now you move into some heavy brush and after wacking down a few sappling all at once, your momentum is used up, the cutter begins to slow, as a result your pressure climbs to max (2100) at this point, the maximum engine hp the cutter can draw is being used. and the maximum hp the hydraulic drive can produce is being created. (there are hydraulic formulas you can use to calc how much hp it takes to produce 24 gpm at 2100 psi) and you would know how much of you 75 is left for the wheels.)
However the reality of brush mowing is you wack a tree/brush at you pressure hits relief instantly, (the full hp you can create is going into the cutter) and the cutter usually makes the cut, but losses alot of momentum, then in the next few seconds it recovers its momentum just in time to be ready for the next tree. The more engine you have, and the more pump and hyd motor volume (flow) and pressure you have the faster your momentum (cutter speed) will recover, and the faster you can cover ground. So how fast do you want to go? How much do you want to spend? Only you can answer that.
I have mowed brush with a 753 up to 3“ with 13gpm and 2500 psi.
Then I stepped up to a 773, it puts out 17 gpm and 3000 psi. I resized my brusher motor to run the same 540 rpm into the brusher gear box, but now I have more torque, 56% more. Did it help much, yes and no. In moderate even loads it does, but when I whack a heavy clump, it still stops or greatly slows the cutter, and I still wait.
And that is the curse and the bonus of hyd drive, Because it is better to wait a few seconds then it is to have every ounce of the engines energy tear the guts out of the gear box.
The tree shear now is a different story, that one is all pressure, (there is no momentum) the flow just opens and closes the jaws at X speed, the psi is what “gets er dun“