I was thinking of something based on either a two wheeled garden cart or a modified hand truck. As far as trying it with a wagon (Radio Flier type) on anything but the best surfaces: No Way No How. BTDT with firewood and other loads. It won't work. Unless it's got sides & a wide track they're too unstable.
The terrain a cart can handle is also determined by the width of its track and the size of the trail.
Most definitely. You can compensate some for the track width by paying attention to how you load the cart. We used to use a hand truck to haul trees to the end of the row for loading on a trailer. The rows were running across about a 30 degree hill.