IMO, 41magsub, if your father goes with those larger wheels you found, you'll like it better than a commercial game cart. Those 20" wheels will give you much less rolling resistance over rocks and uneven terrain.
When the hub/centerline of the wheel is above the obstacle it impacts, it natrually will climb over it.
Also, I don't think it would be too hard to make it folding. I assume he's going to weld it up out of square tubing? A couple of holes for bolts and cotter pins, and it should fold right up.