Lots of factors affect HP requirements for hydraulic pumps including temperature (and viscosity) of fluid, plumbing friction losses, pump efficiency, etc. But you can approximate your requirements with these two equations.
this is a common rule of thumb equation:
HP=GPM x PSI x .0007
So 6 gpm x 2500 psi x .0007 = 10.5 HP
Or, another equation sometimes used,
GPM x PSI/1714 = HP
Gives a slightly different answer, 6 x 2500/1714 = 8.75HP
I think the answer to your question is a 10, 12 or 14hp Cub Cadet can easily drive a decent loader at reasonable speed.
For reference, my Kubota BX23's pump is 6 gpm at (I think)2500 psi, and the loader and backhoe work fine.
One more thing, since you mentioned lifting power. You are limited by the weight of your tractor, so adding rear weight may be needed, which, unfortunately, does add wear on rear axle bearings (unless all the weight is in the tires/wheel weights.)
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