You can explore this portion of their website for information
http://www.tokyodisneyresort.jp/en/bfree/
But when I was their in 2014 I didn't see one wheelchair or
scooter, and on their website they do mention wheelchairs but not scooters, so perhaps all they offer for rental is wheelchairs, so most likely you could be the only one in the park with one.
For the terrain I remember their being stairs with elevators always being nearby, and it's pretty flat with mostly light inclines though I think there maybe a steep one or two, and I don't remember any rides having stairs in them or their queues but Tower of terror, though I do remember their being no stairs access which they alternate between with the stairs, so you may have to wait a bit for entry without stairs.
Over all good luck and have fun!