Almost any power of motor can be geared such that you can go up a really steep grade with a heavy load. The lower the power, the slower you will go though.
For instance, the 5 mph you spec'd out, if you want to go 5mph up a steep grade, is a vastly different answer than 5 mph top speed and maybe 2 mph up a steep grade.
5 mph is 2.24 m/s. 250 lbs is 1112 Newtons.
If you assume a rolling resistance of 0.03 (which is pretty easy on hard ground with a decent wheel/tire) your rollinr resistance would be:
1112*.03=33N With a speed of 2.24 m/s, the power at the wheel would need to be 74 Watts. Not that much power.
Now if you want to climb a 60% grade (again hard surface), this would be a very very steep hill that would be difficult for a person to walk up.
The tractive effort portion is about 0.51, and the rolling resistance of 0.03 would combine to a needed 0.54 TE/GVW to do the climb or:
1112*0.54=600N which at 2.24 m/s would be 1344 Watts again at the wheels.
If you want to walk along with this robot, it will likely outpace you on such a hill as a fast walking pace up a hill that steep is about 2.5 mph or just over 1 m/s. This would reduce power demand to 600 Watts which aligns well with that wheelchair sized set-up.
If you geared for peak power at 1/ms, the robot would likely have a top speed close to 2 m/s which would mean you could get by with a single speed gearbox.
If you don't expect to climb that steep of a hill, you could gear it a little faster to hit your top speed, and just know that with steep hills or high rolling resistance (sand/mud), you may run at a high power demand which could risk overheating motors.
you may want to look into electric scooter stuff:
Here is a new kit for relatively cheap:
Though you would need two kits and it would be a bit overkill for the power you need.
Here are some other possibilities more in line with what you might be looking for.