If by learner legals you are thinking of becoming a dealer for a range of one of the up and coming Chinese manufacturers, then I think you would have the potential to do quite well with this as your bread and butter business. Some of the marques such as Jinlun are becoming really popular with a good reputation, yet there are still not many dealers in the country.
The overlanding side of the business might be your main interest, but I think it would take longer to build that side up. Not a problem if your main income was already covered by the smaller bikes.
I wish you well if do go ahead with it and I'll certainly visit you since you aren't far from me, but unfortunately you won't get rich from my custom.
If you think you are too small to make a difference you have never spent the night with a mosquito.