I have no interest in being a business partner, but as someone operating a reasonably successful business in Japan (meaning the business is profitable and I'm getting paid from it, though my accountant does her level best to show that I don't make one single yen) I can offer some advice:
1. Learn Japanese. At least enough to handle issues with the pension office, talk to your bank, talk to an accountant, etc. My Japanese is terrible, probably n5 at best (honestly, I don't think I could pass n5 but it makes no difference to me so I never bother with tests or practice tests), but at least functional enough to handle these basic business issues. While you're at it, learn kanji! Reading is a basic skill and absolutely essential for business.
2. Don't have your visa based on your business. I could save about 50,000 JPY per month if my visa was not a business manager visa. That's an annoying debt to pay each month just for the privilege of doing business in Japan.
3. Write a complete business plan. Not just back of the envelope calculations. See who your competition is. See who your market is. See what your marketing needs are. Walk into this with a firm grasp on reality.
Starting a business is a good way to go broke quickly. Mitigate the potential loss by doing your due diligence in advance. After you have 5 million yen tied up in capital assets is not the time to learn that there are 10 better, more established places offering the same thing you are and you have no USP.