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Punching a few numbers into the calculator on this site.
A few assumed figures that should be somewhat close
frontal area 25ft^2
Cd 0.6
Crr 0.1 (Wiki has Crr for tires on sand at 0.3, but that would requre all of my engine power to do 35, and I can hit 50+ on sand so I went with something more reasonable.)
4000lbs
Drivetrain efficiency 75% (full 4x4 transmission and all)
500w of paracitic load (probably quite a bit low)
All of that gives a requirement of 35hp.
One site listed hydraulic pumps as having 80% efficiency X2 gives a 64% conversion efficiency. So you would need an engine that could put out 54hp continuously to hold 30mph, or up to 27hphr of storage for the 30 minutes.
Efficient, most likely not.
What he would gain would be a butt load of low end torque, exactly what every rock crawler wants more of.
As far as weight, my rig is pretty light at about 3200lbs, with an engine that weighs in at 650lbs (210ftlbs at 1800rpm, and 115hp at 3200rpm). Depending on how he builds his rig and how big even with hydaulics he shouldnt be over 4000lbs.
Edit to add: I am rarely even at 1800rpm, often below 1000. When I do run above 1500 on the trail I am also feathering the clutch to control the speed.
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