Dean:
I did something similar for an FEL. I don't know how to respond to your question without more particulars. For example, if all the hydraulic flow is available, you have about 12hp at the tractor pump. After line and valve losses, motor inefficiency, etc., you may have 9-10hp to the blades. If you will do heavy weed cutting and want to move rapidly, that is sufficient for about a 36" diameter lawn mower type blade. I have about 14hp applied to a 48" blade and it bogs down in medium to heavy cutting unless you move slow.
How will you mount the blade? To get 800 rpm (which seems a bit slow to me) you will need a motor of about 2 1/2 cu in/rev displacement. I don't think you will find any motors of that size that have the tapered roller bearings on the output shaft needed to withstand the shocks of a directly mounted blade. So you will need to either make a separate housing with tapered roller bearings to hold a blade shaft or (as I did) use a rotary mower gearbox.
If you will search this site, you will find at least two such projects that may be helpful: one that I did and another that mounted the mower head on a custom made articulated arrangement. The latter seems closest to what you plan.
Good luck and keep us posted.
