Cashflow is a Stream that feeds the business plan, For you to get started with the Business according to the road map which is your business plan you may need cash to secure the products through buying or manufacturing. Once you start operating you need to manage the flowing in and out of cash in your business to keep the business going. Know what you spend on from the income that comes in and manage your expenses. Pay yourself salary to avoid using the whole revenue that comes in as a blank cheque which may leave you with no money to buy stock.
As for Business plan, that is a guide that will tell you the kind of business you want to venture into, the market you want to sell to, and how much it will cost you, and how you will conduct your marketing and promotion of your product. And question today is whether business plan is relevant...answer is that give it any other name you may want like A House plan that can be called house sketch or architectural designs but at then end of the day it is a guide regarding the house you want to build with measurement. So that document you will come up with for your business will be a guide on how and when to get started and what you need to get started. You need to ask yourself questions that you should answer yourself before approaching anyone to help you. After answering the key questions you can then seek professional help to structure it according to funders specification. Most funders have template on how they want the business plan to look like or to contain so do not deviate from that guideline since your business plan may be rejected due to not meeting the requirements.