E-bike parts. Seems like a good application of something like a Bafang BBS02 (or knockoff) + thumb throttle. Chain drive, plug in a thumb throttle. Could instead use a hub motor, but their starting torque isn't great. Either way, you save having to re-invent the wheel in terms of motor control electronics integration.
geared hub motors are a start, but it's hard to find ones with large enough gearing for the torque of a load going up a steep incline (not to mention roots, mud, etc). a few companies sell powered wheelbarrow geared hubs with much higher internal gear ratios. but i need to figure out what the maximum force required is for a given load at a given grade and then figure out how to determine if a motor can provide that; and whether a custom drivetrain will be needed or if i can cannibalize a bike or other drivetrain (and how to tell if it's capable of handling the load). like someone else mentioned, i didn't think about adding the weight of the vehicle into the calculations
What I suggested is a mid-drive motor, not a geared hub motor. Can then size the drive and driven sprockets to whatever reduction meets the requirements of the build.
minimum specs: working load 300lbs, size about 4ftx4ftx4ft. that would carry 1 large adult person, and is the minimum weight for a wheelbarrow full of soil. (a load of soil can be up to 500lbs; for contractors, a wheelbarrow can carry up to 660lbs or 60 cubic feet of concrete. aware of powered wheelbarrows, want this to be remote controlled)
My buddy was featured on the cover of make Magazine for building a remote controlled mower he put a wheelbarrow on and turned into a remote controlled lawnmower plus dump cart
it should be able to carry a 0.5 m^3 cube and 70kg. maybe optimize for low cost and the ability to roll over some sporadic large cobble?