I'm curious too how about some arbitrary requirements. it'd be fun (maybe useful?) to be able to pack in a cooler or a couple tires so let's say..

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?

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)