If it's in your budget and what you want, that sounds like a solid price to me assuming it's not been messed with or abused.
Searching the internet for "C125 knobby tires" yields just a few builds, but they look wild. Two concerns there are tire clearance, and that most (if not all) of the tires that will fit the stock wheels will require a tube because they are tube type tires. Even though the Super Cub has sealed tubeless wheels, the tire construction of tube type tires is different and not designed to seat properly without a tube, they are also lacking the inner layer that seals them to be air tight, so if you can get one to mount properly it will likely slow leak and need to be aired up frequently. For example this is what can happen with a Shinko SR241 tube type tire without the tube, mounted on a rim that is sealed:
No biggie though, you can just run a tube in them while using the tubeless rim.